Why Did Pirates Wear Eye Patches?

When the light comes from the object reaches our eyes, it passes through a hole called pupil. When it is bright, the pupil size decreases, allowing less light to pass into our eyes. When it is dark, the size of the pupil increases, allowing more light to pass into our eyes. Thus, it helps us to see better in dark. However, if we are suddenly dropped from a bright room into a dark room, our eyes take some time to adapt to the darkness. Hence, the pirate wear the eye patch on a single eye to adapt darkness. Now, while fighting, the pirate have to go under the deck where it is quite dark. He will just remove the patch from the covered eye which is already adapted to darkness helping the pirate to immediately see in the dark.