Yes, as noted, you can reserve a specific site at Watchman. Also some others like Bryce, Arches Devils Garden. Reservation.gov sometimes offers that option.
Other NP CGs may be no reservations at all like Devils Tower NM and Chaco Culture NHS.
Some parks have concession operations, for example Fishing Bridge in Yellowstone, where you reserve space and may not be assigned a site until arriving. That turned out well because one of us was having arthritis pain and difficulty walking, and we were given a site a short walk to the office/laundry.
So it's all over the map.