They are trying to preclude people from paying for a site that they don't intend to use until a day or so later. Some have suggested for first come/first serve sites to go in on a Thursday and pay for the site including the extra day but don't arrive until Friday. The feeling is this gives locals or those who can sneak down there a day early an unfair advantage over those who can't get there until Friday. It's also designed to cut down the number of no-shows. Sometimes a no-show is unavoidable and sometimes it's people reserving the site even if they aren't sure they are going to use it.