It is supposed to help reflect sunlight away from their face in order to see better, but nowadays people wear it regardless if it is sunny or in a dome.
I haven't the foggiest. I played a lot of football and one day a teammate came up to me during a sunny day game and wiped on some of that crap under my eyes. After the first series the trainer was spraying water into my eyes to flush them out.
stuff was useless to me, I didn't get it