Ytdyklly

For the same reason we watch movies. It's cheaper and safer. Because we actually don't want to BE in war theatre. Or we cannot access Buckingham Palace and pet the Corgis.

And then we have "influencers". You know the instagram type. Smoke and mirrors (which itself hide a whole explanation on why does this things).

The bottom line - the fact that the thing exist don't mean it available for everyone.
Why have a model of space Rocket X when there is already existing physical one?

It takes a master blacksmith $t$ time and effort to make a really good sword. It takes a wizard $2t$ time and effort to make a similar sword. Why would the wizard do it?

It takes a master baker $w$ time and effort to make a really great pie. It takes a wizard $2w$ time and effort to make an acceptable pie. Why would the wizard do it?

Because right now, I'm up the Khyber pass without a paddle. Nobody has eaten in a couple of days, the bandits are on my tail and I've just found a wizard. The wizard can make both, without having to be a master of either. While it may be harder, slower and less efficient to use wizards for manufacture, what they lose in efficiency they more than make up for in versatility. You don't need a master blacksmith, a master baker, a master carpenter, all you need is a couple of decent wizards.

Previous answers already stated the cost of creation in materials versus the cost of magic. I would add:

Versatility
You're a spy than can't bring his tools/weapons in a specific location? It's not a problem if you can make them appear out of thin air. You're a warrior in the midst of battle, needing a weapon? Materialize it. Your cart wheel broke down and the nearest village is miles away?

In short, (depending on how your magic works) it's possibly faster and probably easier to materialize things than to carry them around. You don't have to plan for every possible needs you may have, since you can create the one thing you need on a whim. A lot of contextual applications will occur.

Illusion
You can't tell it's a magical object unless you're trained? Con-artist magician. Sell fakes of valuables, pay for your food with fake gold and lie about your identity with fake documents. (Note that there is probably legitimate purposes to this aspect, besides being a huge fraud). Also, become a licensed expert magician, to detect fake valuables, fake coins and fake documents.

Time
How long does it takes? Maybe a smith takes a lot of time making a sword when you can do it in mere seconds. You won't be a industrial revolution by yourself, but it seems extremely useful.

A small point I'd raise is the duration and cost of the item. Just produce enough copy of... let's say sword for an army, and the countryside will soon be littered with discarded ones. Children will be playing with them, villagers will use them as fences, they will smelt them, etc, etc. A strong supply of an item decrease its value. If you make it too durable, then virtually everything loose value, since you can make as much as you want. So either they disappear after a given time, or creating one item is taxing enough you can't do it so fast than a trained artisan could do the same during your recuperation time.

If given the choice between getting off the couch to get a beer from the fridge or summoning it, I would summon it. If the only cost is skin cancer, I would have my roof made of glass. Probably summoned glass.

During the middle ages this is even more so because such magic wouldn't be used just to solve first world problems. Some resources were difficult to procure. Clean, fresh water comes to mind. Good iron ores, or better yet, quality steel would be great for just about everything. Fresh fruits in deserts and at sea would save people from scurvy.

People already lived less in the middle ages, so if the only cost attached is a higher risk of melanoma, your mages are actuallly getting a huge profit from life.

There is one simple reason. Ownership. You new clothes, food, wine, beverages, a castle or two, maybe even a magic sword or so, and you don't have or own them.

Solution: Use your magic to manifest them.

You have abolished your lack of medieval consumer goods with your magic. Also, if you have manifested valuable material that is indistinguishable their physical equivalents you can sell them and get rich.

Manifesting material objects by magic solves ownership and wealth problems. Sorry, gang, once again economic reasons gazump the rest. Another triumph for the dismal science.

I can go to IKEA to buy a couch. I'm talking about the real world right now. However, if I could materialise a couch without the entire trip to IKEA, arranging a transport home, and then putting the couch together, I'd probably go with that option. I am even somewhat lucky to have an IKEA close-ish to me - it's only about two hours away. In one direction. And they do deliveries, too.

I doubt there were as many IKEAs in the middle ages. Suppose I wanted to get one Ye Olde Couche

1. I'd have to go visit the guy who makes stuff from wood who is two villages away.


2. I'd have to make a special order because I doubt he has any Ye Olde Couches at hand. He'd have to make one especially for me


3. That might take, say, two weeks. I'm probably being optimistic here.


4. He might just do a single type of couch. He never needed to learn to make another.


5. He might not even offer a variety of woods to choose from. He'd probably have whatever is in the local woods. So, if I want walnut instead of dark walnut, because it matches my Ye Olde Drapes better, I probably don't have the option.


6. I still have to transport this to my home across two villages.

And that's a simple example. What if I want, say, a trebuchet or some armour. There might not even be a decent armoursmith within two days travel from me. Or a...trebuchet-smith. Or whatever the person is called.

A conjurer who can produce any sort of good would be extremely sought after. Want a couch? A trebuchet? An armour? The conjurer has got you.

Let's assume that making a complete object is very hard or impossible via magic. Perhaps it requires very intricate knowledge of woodcarvery, and trebuchet making, and armoursmithing on top of any magic study, to be able to produce all of them. Or perhaps complex creations are simply out of the question. However, if that's the case, let's assume that the conjurer can still produce the raw materials. That's still a huge boon. Being able to get high quality materials to build what you need for them is still better than sourcing these from, say, the neighbouring kingdom. And you might be at war with said kingdom. If the conjurer can produce slightly more complex materials like alloys or a gunpowder mix, that you don't normally have access to, it's even better.

Whatever the case, the point is that even if you could procure whatever the conjurer would magic into existence, that doesn't mean it's easy. Or as easy. If you have iron but not coal or decent wood to burn, you'd have a hard time forging swords. If you have something to build good fires with but no iron, you still have the same problem. Wars have been fought over salt which was a valuable resource back in the middle ages.

Being able to provide enough food pushed humanity to build the civilisation we have today. Can you imagine not having a lack of any resource?

Because not everything has the same value-to-mass ratio. Making a handful of dirt by magic is certainly more work than scooping it from the ground. But making a bar of gold might be a little cheaper than mining it. So you might have goldmines that are really just dozens of magicians churning out gold from the noonday sun.

Also, as Chronocidal mentioned in their answer, some objects require a large amount of skill to make by hand. Why spend days expertly crafting a watch when you could summon it by magic? A watch has much more value than its component ingredients: the hard part is intricately cutting the tiny gears. With magic, that's much easier.

An example from Paolini's Eragon is where Nasuada instructs the Varden's magicians to make lace by magic, because the value of lace comes from the difficulty of producing it, not from the raw materials themselves, so you can make a much larger profit margin if you cut production costs and use magic.

In some cases, crafting complexity: A magically gifted artisan of clockwork may find it easier to sketch out intricate designs, and then manifest the result directly from magic, instead of having to craft tiny gears and jewelled bearings, then carefully assemble the entire construction without getting any specks of dust in it...

A material required for constructing your facsimile may be rare, expensive, or hard to work/produce - how much magic would it take for a jeweller to cut facets into a diamond? And, would it 'cost' less for them to simply manifest a pre-cut (and completely flawless) stone?

Finally, you may also be able to deliberately distinguish your magical copy in some way - taking the magical clockwork example, you may be able to magically manifest a mainspring that provides a near-constant force for the first 90% of its output, then drops off for the last 10%, in a manner superior to even modern going barrel

Ease of use, entertainment, survival, special construction?

Lets say your watermain broke. You can create a quick temporary stopgap and then get "real" stuff and call professionals to close it.

You get stranded in the cold. You can create something like matches to light a fire or temporary clothing to keep warm.

Someone tries to rob you with a knife, conjure a loaded crossbow and rob him right back (or bring him to the authorities, whatever works).

You want to impress someone. Some light indoors fireworks could work, or you conjure a flower.

Need a quick repair but too lazy to get your tools? Conjure some perfect for the situation and done!

Need to pour concrete but removing the support might damage it? You are in luck! Our mages conjure up the supports, and when it's hardened enough they just let the supports fade away! Theres a lot of specialist construction that can benefitnof this kind of production, especially for filling molds that now need specific angles so you can pull it out after hardening.

A better question might be when you wont be using it!