this form of advance reservation, especially of prime camp sites, although understandable if it is only the preceding night perhaps, but becomes plain selfish and disrespectful if it remains unused for more than one night - this is common just before school holidays at free coastal locations (although these locations are quite rare)
it is reasonable to reserve an adjacent site for fellow campers who are coming later that evening or next morning when you have arrived early - this is a very common practice and is generally begrudgingly tolerated as long as it is within reason. It is less likely to be tolerated by self-entitled rich people with $300,000 caravan set ups who also want to set up and reserve an adjacent site for their rich self-entitled friends on sites you have reserved, and who do not want to lose face by failing to do so, but these people perhaps have more to lose if they push a dispute too hard.
if you are the first to arrive of a group of 20-30 campers who are arriving later that day, then most other campers would accept this if you explained it nicely and will find another location, but there is a risk of an altercation if you have taken their favorite annual camp spot and you may have to concede to maintain peace. What they would not be happy about is if the group is not coming for 48hrs or more.
the reality is that at popular camp grounds on long weekends, the camp ground is likely to be full by the Thursday night of a long weekend - often with many campers reserving adjacent spots for family and friends who can only get there after work on Friday. For such occasions, a large group is better to select a less popular camp ground further from a major city to reduce this issue.