We use them on our spotlights when we call varmints. They are kinda like a reflector. But really it just keeps the spill of the light from killing your night vision.

I also have them in the garage with stuff that likely should just be thrown away. Filled with a little concrete...or better yet the smaller sizes, make great anchors. The plastic keeps them from banging up your boat/kayak. Just put a loop of rope/wire/whatever in them before you pour in the concrete.

Say, I think I might need to go drink some more coffee.