For comprehension. Why have genres of movies, or of music, for example? Why pigeon-hole a superhero, what is a Speedster, what is a Brute? Why categorise them at all? Stereotypes give people a quick idea of what you're referring to. If I say my character is a Rocketeer, for example, you know I'm talking about someone with a jetpack and trousers. Pigeon-holes are for comprehension.
We're not all imaginative. In fact, I'd suggest that most of us really struggle with imagination. There's no such thing as creativity, instead we combine existing ideas - we stand on the shoulders of previous mixers. As Picasso said, "good artists copy, great artists steal". I think people will benefit from having ideas presented to them for them to add their own ideas to, rather than give them a blank page and expect divine intervention to provide a miracle.