An organization that has almost all of the worlds most powerful people? Maybe a bit more than being a temp agency. I mean they could effectively take over the world without any problems and yet in most stories they just exist for MCs
From what I understand, they're basically in between freelancers and people who need stuff done. Though sometimes they'll also have some sort of influence over the freelancers working for them, I guess because it's harder for people to get work without going through them, or because they have political power due to the freelancers being pretty strong relative to guards/the army.
I suppose I feel like it's a bit overused? Like, there are other, less systematic ways of having the protagonist encounter problems that need solving or sell the loot they got from hunting/exploring.
it depends on what kind of guild we are talking about. Are we talking about craftmanship guilds? Mercenary guilds and so forth. As for craftmanship guilds, they are more regulatory bodies. They dictate the quality of the products one can sell, as well as who can even sell those products, and to whom you are even allowed to sell, + set up prices. To keep it short, they are kind of monopolies.
Depending on how your fantasy world is built, and how the guilds are (for example, in my case local and kingdom guilds coexist), guilds could for example act as "Fantasy amazon one-day delivery, which wouldn't be uncommon if they have a wide network).
The dynamics between kingdom guilds and local guilds could be even more interesting since both might have clashing interests, for example.