Personal preference is what it comes down to. I don't like the restriction of reservations for just an over night stop because I don't know where or when I want to stop exactly. We are retired so we don't have time constraints so I can deviate off route anywhere I want to. Never been shut out of a campground.
We have pulled into campgrounds for a night and have extended our stay many times because it was an area we wished to explore a little more and the CG was nice. Our primary travel area is the southeast from NC and all the Gulf states to TX. Never a problem getting in somewhere.
We use the RVPARKY app when on the road to find places to stay. We start looking around 2pm for a place to be off the road around 4pm. Easy enough to make it by a phone call. For stays of a week or more we do make reservations for our destination.