Catharsis, and I've long believed that there's so much rapefic because a lot of people get into fandom when they're teenagers and the threat of rape is (hopefully) something they're just starting to become aware of, and writing about it with trusted characters is their way of working through the fear of the threat. It's a controlled environment with familiar characters that allows them to explore not only a) how one would deal with it if it happened to them, but also b) what would motivate someone to hurt someone like that.

I remember when I was a teenager....I wasn't even aware of fandom as a "thing" but I still made up stories involving musicians I crushed on or book characters I liked somehow dealing with rape, and this is probably what was behind my motivation.