Like Barbara said all the rules is to make rush fair for both the chapters and rushees.
But the biggest reason for the rules (imho) is also the biggest difference between IFC and NPC.
NPC's goal is for every girl on campus to go through rush and join a chapter. They don't care which chapter as long as it's NPC. (That's how it was explained to me by my sorority's National VPM.)
NPC does also work to establish guidelines for other concerns, like alcohol. But the number one reason NPC exists is to keep NPC existing.
IFC, on the other hand, is in existence pretty much only to establish guidelines for alcohol, hazing, etc.
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