I can't imagine not doing that. Say you have 1500 PNMs and have to cut 70% after first parties. Can you imagine what that cut session after first parties would be like if you hadn't pre-cut?
At most campuses, there aren't that many women you have to cut for grades (the easy part) so somehow you have to figure out how to select the rest. Some sororities have their alums in the big cities handle a lot of it.