1. To live out the sorts of things I couldn't really do in real life, for all sorts of reasons, other than than the simple 'there's no such thing as goblins. This includes being heroic (which I know [second-hand, fortunately] to be a very bad idea to try and seek out), wearing armor and having swords (which I do every year at the local ren-faire, to a certain extent, but don't have the time or resouces or craziness to pursue more often than that in real life), making journeys through the wilderness (I just like my computer too much for that), and killing bad guys (because we all know how horrible killing is in real life).
2. To take part in epic struggles. Nuff said. However, I'm the one presenting the problem more often than the one doing the struggling.
3. Because I got sick of MMORPGs, and all the good singleplayer ones are too short.
4. To hang around with my friends, and make really, really bad, spur-of-the-moment puns. I've got Skill Focus: Punnery, and two levels in Master Punner, after all. I can inflict 2d6 sanity damage if I get a critical pun, you know.
5. What other hobby justifies a 20-sided, 34mm-wide, half-pound lump of metal for $15?
6. Post 300! I am a golden god!