Plus there are many use cases where it would be nice to have professionally made, unique art, but the costs of creating it would simply be too high.

For example, a group of friends sitting around playing D&D, would not likely commission custom artwork for each of their characters. With tools like stable diffusion, you could have a set of several images of each character, as well as for NPCs, landmarks, runes and so on, all in an afternoons work, with some knowledge of the tech.