It depends on the bike and the design of the windscreen. I would say that most are designed to shelter you from the wind first. There might be a slight amount of protection from sand and rocks getting kicked up but most motorcycle windscreens don’t provide the rider with full coverage so you might still get drilled in the helmet by a rock even though the aerodynamics are directing airflow above your helmet.

What I found interesting when I was a new rider is that most windscreens aren’t even designed to be looked through. The rider typically looks over the top of the windscreen. I have ridden some cruisers with giant windscreens that are designed for looking through and they’re very annoying because as soon as a bug splatters on the windscreen there’s no way to clean it without pulling over to bust out a rag and some furniture polish.

I’d say the main benefit of a windscreen is comfort. On the highway it can be very tiring to hold your upper body against the wind without one. It also helps prevent buffeting of the helmet and it reduces noise in the helmet quite a bit so you can hear traffic or your music more easily. The downside of the wind protection is that it reduces airflow at low speeds which sucks when it’s hot as balls outside and you’re sitting on top of a hot engine, catalytic converter, and exhaust. Some fancy bikes like my R1200GS have adjustable height windscreens so you can choose between protection from the wind and rider-cooling airflow.

I think those are managed by the National Parks Service which is separate form the Forest Service but I could be mistaken.