Being one of those who joined one of these "Private Campgrounds," I'm comfortable explaining at least our reasoning.   But I'll note that many of these campgrounds target "full timers" or folks who live in their motor homes all year.

We belong to Western Horizons, with a "home park" about 2 hours from our home.   Yes, there's an entry cost (depends on how much you buy into) and an annual fee, at least until I can find three other folks willing to join, then my fees are waived.

WH is around 22 or so sites.   We pay $6 a night for stays at "non-home" WH parks, though there is an option to buy that out, too.   We can stay two (non-peak) or three (peak season) weeks in the "system" then one week out of the system.  Sounds complicated, but we really don't bother with that much, not being full timers ourselves.  Coast-to-Coast is another add-on option (we passed on that one).

So why do it?  For one, the amenities at these parks exceeds anything we've seen, other than a few of the first-come-first-served state owned parks.   We can always get a site.  And the real reason is our "home park" is near some land we're working to improve in hopes of building there some day.

Jon