I don't know.
Wes asked me today, and I'm really not sure. I think I like doing them because they make me feel special. Outside of my running group, I don't know too many marathoners, and of the other marathoners I do know, I'm faster than almost all of them. Plus I really like my running friends, so it's fun to hang out with them and work toward a common goal.
As far as a marathon, well, it just became the logical next step. I ran a 10-miler, then I ran a half marathon, and I had such a great time that I had to go for the full. It's only recently that I've come to appreciate how different 10-milers and half marathons are from the full marathon distance.
Of course I also have a wicket sweet tooth, so marathons help with that. I've heard some people lose weight while doing them, but I wouldn't know anything about that.
The diet starts tomorrow though, so for my next marathon I'll be totally jacked and possibly a few pounds lighter to increase my aerodynamicity.