Motivation. A frat (or at least a good one) motivates you to be more. You are surrounded by other guys, and while some of them just want to be dude-bros, there are a lot of guys who are going somewhere and help others aspire to go somewhere. Sure you can volunteer, donate, and excel outside of a frat, but statistically? Not as many people do well in school, and only a minuscule portion even consider donating or volunteering outside of extra credit opportunities.
I was part of SAE at Oregon State University during my time there. It was a fantastic experience. I made a heck of a lot of friends and surrounded myself with good people. A good frat is a family. I had access to a whole lot of resources that I wouldn't have had otherwise. If I was having problems with my coursework, I could go to one of my mates in the frat who was further along than me and work through it. They kept 'test files' (old tests from just about every course on campus that you could take out, study, copy, and take as practise. Yes, the University knew about it, and yes, they were ok with it. Teachers are told not to reuse material anyway.).
People on the outside look at Greek life and only see pretentious assholes trying to assert dominance over 'geeks' or 'nerds'. Yes, that is an unfortunate part of college. No, it is in no way limited or concentrated in Greek life. In reality, Greek life can be a fantastic lifestyle for someone looking to get through college. Chapter house life was one hell of a lot more peaceful than dorm life for me (and one hell of a lot cheaper than dorm life and the mandatory meal plan or getting an apartment). Statistically, Greek house parties were less likely to have any violence and had VASTLY reduced possibility or drug usage (shit would get you kicked to the curb in a heartbeat). Besides, one heavily enforced rule in my chapter? If your grades dropped below 3.0, no parties for you AND you had mandatory study hall every single weekday.
TL;DR - People like to generalize Greek life based on a few asshole jocks wearing frat muscle tees. In reality, much of Greek life helps to encourage, support, and motivate its members.