Social clout basically. A whole lot of people want to get into the partying and casual sex side of college & frats tend to throw a lot of parties due to having money to throw around (due to attracting wealthier students), and thus control who gets into those parties. Also, you instantly get a community by joining. Doesn't mean it's a great community, but lots of guys are pretty bad at making friends.

That said, I doubt it's a great experience for people without money/connections. And I wonder how much of a difference it really makes, because if you're well-off, social, and decent-looking, you're probably going to do pretty well in the college casual sex scene regardless.