For an additional example: We a few days ago camped at an ultra-popular and beautiful large lake with a huge beach that we could drive our RV right down on close to the water and then rent a PWC for myself and the grandkids. Using Reserve America, we had easily made reservations two weeks ahead of time for a couple of desired days at a campsite close to the beach. We canceled the reservations while on the trip two days before arriving and lost some money due to the late cancellation (a long story).
HOWEVER, the next day we frantically tried to re-reserve the same campsite right back. We could only re-reserve the same campsite for the 2nd of our original desired days, but had to take our chances and drive to the park to try and get some kind of campsite for the 1st of our original desired days. When we got to the park, we got our original campsite back on a first-come first-served basis for the 1st of our original desired days. Hence, in the end we got our original desired campsite for the original desired days - all happening in a hurry while on our trip through reservation, last minute cancellation, last minute re-reservation, and first-come first-served drive up. On top of all that, there were several other open campsites open right close to ours - but even closer to the beach than our campsite.
However, usually our camping style does not require reservations .... and certainly not reservations much ahead of time. We are rockhounds, bird watchers, nature observers, ghost town visitors, etc. - so we don't camp where most others want to. We're not afraid of drycamping and have no problem running the A/C in our self-contained rig for hours to keep cool in summer weather in out-of-the-way camping spots. We don't need cool coastal areas or cool high altitude mountain areas to enjoy summer camping. In the winter is when, if at all, we hit our coastal areas for ocean camping without reservations.