Yeah, usually it's used because the saddle slips forward. However, I think the reason so many endurance riders use them is because a lot of them use treeless saddles, which tend to move around a lot more. That would be a good reason to use a crupper as well. A (non-endurance) friend of mine ALWAYS rides with a crupper and breast plate because, with both of those and horses with good withers, she doesn't have to have the girth as tight. She frequently describes to me how she used to ride and be able to stick her whole arm through the cinch, but that saddle didn't move since the breast collar, the crupper, and withers kept it in place instead. She liked that because she felt the horses had a lot more room to breath.
I was never there and so that is just her account and opinion, but if it did work so well, I've often wondered if something like that would be an advantage in endurance?