The camp site creates a refuge for the player, who can rest there for free and for as much time as they want. Restores 8 sanity per day for 5 days for a total of 40 sanity.
Since Alpha update 30, Camps can now be packed up and will return to your inventory, allowing you to use them again and again. It takes 3 days to establish the camp, 5 to fully rest and another 3 to fold it back. If time is not a factor, it's the most useful sanity-restoring items.
The Camp site makes travelling much easier, and can be good, especially for new, inexperienced players.
It is a good idea to keep sanity 30 or above before resting in a Camp Site to prevent the bad events from happening to your party.
This also stacks with Fresh Air Fanatic perk to give 65 sanity per rest instead of 40.