Fraternities are really interesting organizations because they are "social clubs". Social clubs don't really exist anywhere else in society in the same way; they are usually much less formalized.
I'm in a smallish (50 or so brothers) Jewish fraternity. Overall, if the circumstances are right, I think fraternities and sororities can be really good. On the other hand, they can also be bad influences, and the stereotypical frat dude at my university is a real d-bag. At least some of the fraternities (like mine) I think most of the people are well intentioned and good spirited. We don't do any hazing or anything like that. Some things to watch out for in the fraternity system are: elitism, hazing, drug (mainly alcohol) abuse, and general distraction from school. Assuming the fraternity is a generally well-intentioned, good-spirited (morally and ethically) organization, it offers a lot of benefits to the participants.
Some benefits:
1. You have a really nice way of meeting a big group of people, really easily. Also, this allows you to meet more females, future connections, etcetera.
2. Imagine the best party you can throw with 2-3 people. Now, multiply that by 20. Then you have a fraternity party, and throwing your own fraternity parties can kick major ass. If you have 50 people working on a party, you can do much more than you could by yourself.
3. Chance to work on your social and interpersonal skills. You probably won't be best friends with everyone in the fraternity. There are just too many people, and a lot of people that are very different from you. Most people aren't super compatible with each other. However, you CAN be respectful and understanding of everyone, regardless of your level of friendship. In this way, you get the opportunity to learn to live and work with other people's differences.
4. Good leadership opportunities in the form of: (a.) personal leadership experience that you will acquire, and (b.) observations of the leadership qualities of others. The latter aspect I feel has been very valuable for me, as there are definitely some people in my fraternity who are just natural born leaders.
5. Definitely other benefits to be had...
The initial experience I had of joining the fraternity was really awkward for me because it felt like a totally artificial way to make friends. I felt like the fraternity was trying to force friendship on me or something. So, the fraternity can be a pretty unusual experience at first. With time, I became more relaxed and my expectations become more laid back. Now, I am really thankful that I have my fraternity as another outlet for personal activity. Hope this helps.