When we travelled across the continent and back in 2015, we used PPA a lot. We only had reservations in a few spots (Niagara Falls, etc oviously NOT PPA) and for the rest of the time we winged it. Around noon we'd estimate where we would be by 3-4:00 or so and then either open the PPA app or book (they send you a book the first year then you pay for it if you want an update)and see what PPA campgrounds there were in that area. Sometimes there were none unless we went well out of the way; sometimes we had several choices. The App/book gives you a summary of what they have (pool, etc) but usually we were just looking for a safe place to overnight. We'd choose PPA for $15 over a Walmart. If we had to pay over $30 just to sleep, we'd sometimes avail of Walmart's generosity, though my DW isn't fussy about that.
Some people here seem to think that PPA campground are exclusively PPA. That's not true. Many cg accept many different discount cards. We've often stayed at cg where the people in the site beside us were paying the regular rate - twice what we were. Or they got $3 off for GS when PPA gave us $15 off the $30 rate (we have GS as well). We've never felt we were given lower quality sites because we were getting a discount.
On our cross countries trip in 2015 I figure we saved about $400 in total using PPA. We could have saved more had we been willing to go out of our way or if we wanted to stay further from wherever we wanted to visit. Last year, I'd estimate we saved about 2-3 times the cost of membership.
We don't always use PPA. There's a cg we like to visit nearby that doesn't accept it. But when travelling, it's the first place we check; only looking elsewhere if there is no PPA in a convenient location for us.