In all our years of travel, we very seldom made advance reservations and never had a problem finding a site for overnight. As already mentioned, if we were planning on staying overnight near a popular tourist destination, we would call ahead and see if a spot was available. Once in a while we would even reserve a spot - especially if we were planning on staying more than one night.
The reason for not making them was that we were never sure exactly where we would be wanting to stop. If we knew we wanted to stop at a particular campground then we would sometimes make a reservation.
Another reason is that most campgrounds want a credit card to reserve. We were not comfortable putting our number out there for a stranger unless it was the only way.
This was our preferred method of travel for over 40 years.
Barney