The advantages to the map books is that one need not be on line (important to Full Timer or multi-week traveler in remote area) and if the CGs are marked, they are marked right there on your map.
However armed with those sites you can print out the list of campgrounds in the area you might be in. I plan on carrrying with me this summer the list of Forest Service and BLM campgrounds for Washington and Oregon. Mostly the western part of both states. I also have mapping software on my computer that I can plug in the approximate locations then I have the whole thing. I can print out ahead of time if I like.