Anyone actually using see-through red dot optic lens covers? I'm talking about the ones that come with Aimpoint T2's (and similar) that are see-through. Whats the point if the optic is supposed to be bomb proof? Sacrifical layer? Keep it clean?

Maybe I have too much time on my hands, but I was sitting there looking at it and realized I'm giving up a tiny piece of vision and could not really find a reason why...