You can get by with no CG reservations except for a couple of exceptions: Holiday weekends in popular areas. And major National parks (and some state parks too). If you don't have a reservation for Fishing Bridge in YNP your chances of showing up and hoping for a spot are doomed to fail (for example).
You have to enter a lottery months ahead to get a spot in Yosemite, especially one in the valley. And if you don't have a spot in the valley you will be doing a lot of driving each and every day. If you had to stay outside Yosemite then I can see why you weren't too happy about the park.
The same applies to all big parks and a lot of smaller ones.