Almost every job listing that's tagged with copyediting (or proofreading) is also tagged with writing of some sort. Seems weird to me. What am I missing?

Are they seriously looking for people who are good at editing their own writing? I mean, that's not really a thing, right? It's a big world, so I'm sure some people are good at it, but probably very, very few.

Or are their places where two or more people edit each other's writing?