Lake Michigan shoreline, you need reservations. Same with MN north of Duluth along Superior. Same with Yellowstone, Yosemite Valley, and other extremely popular places. But there are scads of other locations where one can get by pretty well without reservations.
I hardly ever travel with reservations; it's a loss of freedom IMO. I want to travel 'foot-loose and fancy-free'. But when I went to Yosemite, I did manage to reserve sites in the Pines CGs for a Friday and Sat. in July, by watching for cancellations; then on Sunday a.m. I drove to Bridalveil CG where it's first-come-first-served, picked out a site from among several empty spots, and stayed for 5 nights. Similarly, I showed up at McLain SP on a Sunday mid-afternoon in late July and they had 3 open campsites, one of which had a great view of the lake and became mine. I had to move to a different site for the next night, though. My point is, even in the popular places reservations aren't always absolutely necessary... but it's good to have an alternate CG or two in the vicinity as potential backups.