I have seen many people training while wearing these type of masks. To keep it simple, the theory behind it is that because it makes breathing more challenging it will improve one’s aerobic capacity. I know people who do long runs and workouts with these masks. I have never used one nor I would recommended. As for the person you saw boxing, I’m not sure if that is a great idea or not. And of course as long as he doesn’t get punched in the face while wearing it ;-)!