I think it's just there to provide a throttle for reputation gain, which has a few small benefits (beyond the obvious of helping curtail abuse). I don't necessarily believe these to be true (I think it's mostly just an abuse throttle), but they're interesting to think about:
First, it means that a user with higher reputation has perforce earned that reputation over a span of days, visiting the site over several days and actively participating during that time. Second, it helps enforce the idea that reputation (and consequently, things you do to earn reputation) isn't the sole focus of the site, that there are other ways to participate which don't necessarily result in reputation gain (but might have badges associated with them, or no "reward" at all).
