There are actually two kinds of alts, and they are so different that it is a shame they are always rolled into one group.
1) The banned user who needs his fix.
2) The person who has an active account, but who also enjoys being sneaky, "duping" people, and has a tremendous amount of time on their hands.
There are a lot of people that fit into category 1. Back before my mind started going to ****, I could have rattled off 15 or so alts that I created over my long and banned-filled tenure of this site. I was a junkie, and as a junkie, I would have been willing to give hand jobs for crack, er, alts. Dalamon is another perfect fit for this category. I know of at least two alts he has made since his banishment, and suspect several others over the last six months. There have been others over the years, and their will always be more in the future. In a sense, it is a sign of a great board and an even greater community.
Category 2 on the other hand is a bit more rare. Like Siro stated, I suspect these people probably have had some sort of background in RPG's, liked to play dress up when they were young, and are typically a little higher on the intellectual scale. It takes some serious time and talent to mature an alt into an accepted poster, especially while running an original account. There is a reason that I always got caught: it's hard to turn this much charm off. Pulling a fast one on people, especially people you know and like, is awesome. In most ways, this is a no-harm-done type of "fast one", and so people don't really have problems doing it -- I'm sure it's a pain in the *** for the mods and admins, however. Here's a fun little tidbit: Two of the top ten (by rep) are both alts of current users. You'll never guess who they are, and you'd die and/or not believe me if I told you. It's all good though, since they don't cause problems, etc. Jackholes like Heaven Harris and ScerpioNJ on the other hand, ruin it for the rest of us.
FWIW, both categories suffer from Small Dickitus. Our balls on the other hand...