A few things. It makes for no distraction in the eye you’re not using, while still allowing you to have both eyes easily open with focus where you need. Keeping both eyes open vastly reduces strain on the facial/eye muscles.
Sometimes a white opaque shield is used, on the theory that allowing light through won’t have potentially negative effects on iris contraction. I can’t speak to the validity of this one.
We use tape over the left eye of my youngest’s eye pro when he’s shooting a rifle. He’s cross eye dominant and scotch tape on the glasses is cheaper than a left handed rifle.
Sometimes a white opaque shield is used, on the theory that allowing light through won’t have potentially negative effects on iris contraction. I can’t speak to the validity of this one.