All you ever need to know about the purpose of fraternity/sorority systems is explained by the prairie chicken

I shit you not. All scientific like.

lekking is relatively uncommon and mainly confined to birds – although some fish and insects do it. In a lek, basically, a large group of eligible and horny bachelors get together in the same place, fairly close to each other. They then each do some kind of individual (although, rarely it can be coordinated) mating display. They may show impressive feathers, or brightly colored throat sacs…They may call out, do an ornate dance, jump up and down, or even jostle or fight each other. Their main goal is to show that they are the biggest, strongest, most healthy males in the group…
In any case, by whatever definition of sexiness this particular species uses, the females in the area all gather around and watch the males in an attempt to determine who they should mate with.

Or maybe it was football and cheerleaders. Something like that.