As mdamerell pointed out, the whole purpose from the CG owner's point of view is to sell unused space by offering a discount. Hence restrictions on use during the periods when they can be expected to fill up be it day of week or seasonal. Most do not offer the discount if you want to make an advance reservation.
We find PA very useful in some areas, not so much in others, but it always more than pays for itself. When we start out on a trip I start with PA and look for places along our route, make a list of them with locations and phone numbers. Then in mid-afternoon, once we determine where we want to stop, I will call ahead for that evening.
We have found both good and bad PA campgrounds. Some really nice mom & pop resorts that we have returned to, some that we have driven past and said "Nope." A fair number that have just hook-ups in an open area with no manager on site, just a drop box, still good for an overnight.
I always follow up the PA list with a look at
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