I think the reason, according to historians, is because it took a while for everyone to assemble in one place and cast their final votes (which I'm pretty sure is why the elector vote is a month after the election). News traveled slowly at the time but it was possible that you might learn something of the President-Elect during the period after the election that might make him illegitimate in your eyes. I think that's the general justification for why the system exists, and please correct me if I'm wrong.