Last weekend we camped next to a prime spot that was empty because someone didn’t arrive to use their reservation. It was in a US federal area, and one of those campgrounds where the sites are reserved basically as soon as they can be, six months in advance. The ranger said that most forest service and park service campgrounds don’t have the staff to monitor no-shows. I think there used to be some system where you’d’ forfeit the site, but either my memory is wrong or things have changed. The rules are a bit different everywhere, but upshot: please do cancel your reservations if you won’t be using them, to make sure someone else can enjoy the site. Don’t assume it will be automatically forfeit to a first-come person.