Biggest reason? My comp was overheating any time I undocked for long so I needed something to do while waiting on the new computer to arrive.
Actually, that's not the biggest reason at all. That's just what got me really stuck in. Prior to that, it as just fun.
Also, it's an escape from reality, and courtesy of a chronic health condition, reality will occasionally choose to really, really suck. Last time I got really sick, I spent as much time RPing as I could manage. Fuck you, real life.
More philosophically, I've always enjoyed playing pretend, overactive imagination and all that. Roleplaying is just the grownup word we use for it. Let's us act a little more dignified about it.