I believe the reason the frats do this is create these videos...example:
http://www.youtube.com/watch?v=-S0RHow7sXU
And then they use them to promote their houses for rush events. The sororities use it as an opportunity to promote their unity by dressing alike in "round up gear" which are coordinated outfits from wind shorts, bike shorts, socks, baggy tanks..etc and they have certain outfits they wear each day. The frat guys buy this merchandise too....but they buy it blind as the actual gear is a highly held secret until the day it debuts. Beyond gear there are croakies, flasks, medallions, beads, the list goes on and on and on. I personally believe houses love it when their jerseys show up on the rap stars or when the singers wear their medallions, etc.