To meet people at a new campus.
To check out the sororities and see if it's what they're interested in before they actually ask their parents for the $$.
Because they're nosy. (LOL)
Because they think if they keep saying "I'm just doing this for giggles, I'm not going to pledge" they'll stay calmer and increase their chances of getting a bid - which is what they want to do all along.
If it's a noncompetitive rush RFM might not be an issue. Also if she's straight up told one of the sisters that she's just going through for fun and they DO need to cut people and it's between her and another girl, I'm sure that would come up in member selection sessions.