If we're travelling "locally", i.e. here in Colorado or Utah, we often go w/o reservations. However for our longer trips, we almost always do. Why? For one, we always try to stay in the NP or SP we are going to see. If you get online, you'll often find these CG's are booked solid every day of the week during the spring, summer, and fall. If we were to just show up, we'd either end up in a private CG :( or at a NFS or boondocking location some distance from the park we are visiting. I prefer to spend my time in the park rather than "commuting" from some distance away. I've driven hundreds of miles to get to our destination. The last thing I want to do is spend more time every day driving into and out of the parks or whatever it is we are there to see. Plus, we can sightsee along the way and not arrive until evening if that's what our schedule works out to be.
Locally, we can leave on a Monday or whatever and find a place to camp. I will say we still don't get into places like Arches using this technique.