I ran into this for the first time two months ago. Due to some unusual travel issues it would have been beneficial for me to be able to check-in around 11 am, normal checking in being 1pm. The campground said sure but there will be a $10 early check-in charge. Now as it turned out I did not get there until 12:30 and they did not charge me or the other two checking in at the same time.
I had occasion to talk to the manager later and I mentioned that I had never seen an early check-in charge. He explained that it was primarily to try and discourage the traffic issues in the campground created when people were trying to leave and having to contend with others trying to get into their sites early. Made sense to me because I think we all have had to contend with that person who just can't seem to back into his site and your stuck waiting 15 minutes until he finally clears the road.