People mostly want to gain
weight either because yes, if you're not muscular and don't weigh much, you just don't look... 'impressive'. Some fat will go a long way then. Not fat-bastard fat, but some fat here and there, so you won't see bones sticking out. Not a pretty sight.
Most do it for health reasons. According to many 'experts' being underweight will leave you more vulnerable to infections and whatnot. Dunno if that's true though.
Of course I can't speak for the TS, but I don't think I'm too far off with one or both of those reasons
Edit: I just ate 6 sandwiches with peanut butter, a banana, a kiwi, and 1/3th to 2/5th of a big bag of chips. And I'm still hungry. What's wrong with me?? ^_^