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
- I'd have to go visit the guy who makes stuff from wood who is two villages away.
- 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
- That might take, say, two weeks. I'm probably being optimistic here.
- He might just do a single type of couch. He never needed to learn to make another.
- 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.
- 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?