Correct target shooting should be done with both eyes open, and the "non shooting" eye covered with a patch or shield...
This patch allows the user to flip it down when shooting and flip it up when not, for reloading magazines, scoring, etc.
Some shooters believe an eye patch should still allow light to enter the non shooting eye to keep the aiming eyes iris normal. By closing the non shooting eye the aiming eye will adjust for the lack of light entering and you will lose your focus, resulting in poor scores.
I have heard of shooters making a patch that hangs down in front of the none shooting eye, out of `milk jug carton` or the opaque plastic bottles we have here ( not sure what you guys use)