Hey Fun Police,
My answer, as TOPNDR has already said, is "it does both". The most important job it does is to help keep the back window intact. But the side effect is that it protects the camper itself from the worst of the crap, and is cheap and easy to refurbish when it does get wrecked itself. Lastly, it keeps the camper cleaner - the amount of mud that ends up on our stone guard at times means I get less filthy when setting up camp.
Do you absolutely need one? Of course not. My father-in-law has dragged his camper everywhere we've been, with no stone guard. He's had just as much fun as we have had.
THAT SAID, we're going to build one for him soon, because his drawbar and front-facing bits of the camper all need a re-paint, 6 months after we painted it last. On a brand new camper, this would break your heart (especially if you had a powder-coated trailer). When we bought our camper we were considering getting JT to strip it right down to save the $$$s. In the end though we figured we would be better off saving up for longer, and getting it right from Day 1. If it's a cost thing, I would forgo some of the bling (like a 12v setup), and put the stone guard on instead. In fact, that's exactly what we did do.
Good luck!
Matto
