95% of the time I don't make reservations.
I'm not as far along as I expected to be on some days, and don't want to continue all the way to my preliminary intended CG. I can pull over, look at other CGs in the directories (GS, CampWhere, Allstays), and pick something close. With reservations I would have to keep going.
I may find a CG or an area I really want to spend more time at, so I do so. With reservations, I am booked and must push on.
If I really want to camp at a very popular, crowded place, I may make a reservation. Like this past summer. I wanted to camp in Yosemite. Kept checking the reservation system off and on for a couple months, and managed to book 2 nights in the valley on Fri and Sat nights. Then drove on Sun am to first-come CG Bridalveil Creek and stayed there 6 more nights. If I could not have gotten a night in the valley, I just would have gotten an early start one morning and booked into Bridalveil; but with the reservations I didn't even have to hurry up the road that morning.
2 summers ago I was able to simply show up at McLain SP, a very popular park on Lake Superior (MI), and despite its reputation for always being full I was able to get one of the best campsites with a great view of the lake. But the thing is, when I started out on that trip I wasn't sure I would even get to Michigan; I started out on the Minnesota shore and after several days there I decided to move eastward.
Reservations are more trouble than help for me, usually.