I mostly like to write roleplays because I'm too lazy to ever write one whole novel-length story by myself, and I lack so much imagination/creativity that it's nice to have someone there that you can conspire with. I mean, create ideas with. Yeah.
Plus I like being able to focus on one aspect of the roleplay, such as a particular character, and then the suspense that comes with waiting for your partner's replies. I just like being able to write from a character's point of view, and then see how other characters perceive things. Kind of like every day social interactions, except I'm very nervous about stuff like that, while with writing I can be more open about things. Except relationships. Which is why I don't really write romance. I much prefer action, sci-fi, fantasy (particularly of the medieval kind), and especially horror. Since I lack creativity, I'm better at writing fanfiction than completely original stories (because I will tend to rip off other ideas for these 'original' ones, but only if I'm writing just for fun), and I enjoy the challenge of trying to portray the characters as close to the canon as possible, as well as just exploring plot holes and all kinds of other possibilities that were left out of the canon.
I just like writing because it's something to do. Because I had no other way to record all the crazy ideas I had when I was younger that I wanted to add onto canon storylines. And also because sometimes I like to imagine myself in those stories, doing things you can't in the real world. And that sounds so cliche.