Hi Ken,
Most, if not all, sites insist on "breathable" groundsheets, even then some sites will also request that groundsheets be lift off grass pitches regularly to help prevent damage to the grass. You will soon notice the pitches where caravanners have not adhered to the requests, they will be the piches with the large brown rectangle of dead grass.
I prefer to use a groundsheet, find it easier on the feet, you don't bring as much grass and dirt into the caravan. Even on gravel hardstandings I will use a groundsheet if the awning is erected. I use a material known as softex, if you google it you will find suppliers and descriptions. I have used it on site for periods of up to 2 weeks at a time and when lifted the grass has been o.k. After that amount of time the grass does start to grow through it though. It's not at its best in very wet weather, because of its mesh like construction it can get a bit dirty with worm casts etc in really damp conditions.
Although I prefer to use one it's not something I would say is essential. It's one of the items you could hold off buying until you have had a couple of outings, speak to people on site, gather some different views and opinions and make your own mind up.
Robert