Well, mostly for fun. Even though I'm kind of disparaging about BJJ and can't watch the UFC, I have always loved rolling. Watching others do it bores the shit out of me, but it's fun as hell when you're in there.
On top of that, I've also realized that as I get older, the situations I get put in have changed. It's not so many all out fights where fists are flying and there's multiple opponents like it used to be, and I don't get involved as much in said situations as I used to because I don't drink like as much nowadays. I also have a huge fear of ruining the career I've got going with assault charges... I'm far more conscious of this then I used to be.
That being said, I like the throws, takedowns, and locks that BJJ has. They're quick and effective, and you can measure that amount of force that you put into it according to the threat level. The places where I hang out now don't have the "Screw it let them fight" attitude that some of the bars I used to go to had, so things get broken up quicker- the cavalry comes and gets the other people out. And BJJ is much sneakier, and looks way better on a security camera then a combination that knocks the guy out and leaves to chance him banging his head on a chair and dying and me getting brought up for manslaughter.
While there are still many situations that it does not work in, it seems to fit my needs at the moment. It will, as someone said, make me more rounded, which is good, and keep my ass in good cardiovascular shape (which, after the first class, I found to be lacking haha).