To reserve a name, usually. Depends on the person who makes it, really. I made mine to reserve names, however I ended up needing to use it one time. Someone hacked the site so that staff members couldn't log in, and luckily a lot of us had alts. And then I once took a DLoA on this account because of some personal problems, but still wanted to enjoy the site. I didn't want to sign on this account at all, anticipating a PM I didn't want, so I used my alt during that break.
So yeah. That's just my experiences with having an alt. But it's mostly for name stealing. :P