I could be off, but based off of what they said yesterday, to me it seems the tiers are more of a courtesy to the student in terms of Hofstra indicating how much interest they have in a student. It gives you a very broad idea of where on the waitlist you might fall, but given that they said they don't necessarily have to get through the 1st tier waitlist to accept somebody from the 2nd tier, it sounds like the tiers don't mean anything internally.
I guess it could be an admissions committee politics sort of thing, like maybe placing somebody on the 1st tier forces a certain amount of due diligence from the committee before moving on to the 2nd tier, per some hypothetical internal bylaw. Like maybe its so if there's an applicant that part of the committee wants and half wants to reject, this way the for's get their say?