If you have the privilege pack you can pre-book a pitch and it will be available on arrivalWe use them when they're in a location we want to visit. As others have said, they vary enormously. Some are very good - others less so.
We have never booked, and I think its worth noting that, in popular aires where there are no marked-out pitches, such as the very popular and well-located aire at La Roque sur Ceze, it is possible that you could book a pitch, only to find no obvious, available space, on arrival.
If others have already parked and spaced themselves out with awnings etc, or if a large vehicle has taken up a disproportionate amount of space, you may not find room, without asking others to re-arrange themselves (this may or may not bother you). Certainly at La Roque sur Ceze, by late afternoon, the app was showing 6 of 28 places as being available. We were already on the aire and couldn't identify spaces where new arrivals would have been able to park without really squashing themselves in. We saw several other vans arrive at the barrier, look at the aire and drive away. I suspect that we would have done the same.
We can't really see the benefit in pre-loading the card. We just put our card in the machine and top it up sufficiently to get out of each aire.
All-in-all, we like Camping Carparks. While we do not seek them out, we are more than happy to use them.
We reserved a pitch at Savine-le-Lac because the TDF was passing right past , on arrival the day before there were 3 french motorhomes trying to access the aire but there CCP cards wouldn't allow them entry
I rocked up in our then pilote P650 and I could see what they were thinking (We cant get in so this stupid Brit hasn't a chance) well the other half got out flashed CCP card against the reader and hey presto the barrier went up .
We got the very last space and with EHU