Why roleplay, you say? Hmm, that's a good question. I do much less of it these days than I used to.
Like most others here, it's about immersion. The challenge of acting out another persona and imagining how they would behave or respond to various situations has been of immense interest to me. Roleplay is a bit like a math equation: the character personality variables are plugged into various unique equations (situations), and the outcome is a huge unknown. What usually kills that is predictability; I like NOT being able to predict outcomes, otherwise it gets boring. The alternative, especially in MMO country, is ripe with illiteracy, immaturity, homophobia, and internet e-peenery that doesn't really inspire me to keep playing.
And as others have said, roleplay gives meaning to my actions for the character. I don't just run missions for any corporation; I have always chosen ones that seemed appropriate for my characters. I don't just pvp to kill people for killboards, I have found a reason to justify said character's behavior. I don't ever let it limit my behavior, it lends substance to it.
It also lets me be creative in ways that I may not pursue out of the game that I can here. Artwork, writing, game design, these are things that I love to do but would seldom take the initiative to do if I didn't have an immersive universe to play with.