Early arrival.

Management tool. The fees are like some other fees, there to discourage certain things. If you don't have some way to enforce check in times, you will get people arriving well before the people who are in that site check out. If the park is a busy park, there may be no place for them to go, and they will be upset that they have to wait around for their site to be vacated. We have seen where early arrivals start to hover around the people who don't have to leave until checkout time. Some even have the gall to ask how quickly those who have the site until checkout time can leave because they want to get into that site. That is not fair to the people who paid for that site and who have a right to it until time to check out.
When people see it costs money to arrive early, they magically don't show up until check in time.