First, they may have injured an eye, maybe in battle.
But usually it was so that they could see when they went below deck.
They used one eye above deck in the bright sun and then when they went below deck, they moved the patch to the other eye so they could see in the dark hold of the ship.
McAveler, I don't see any plunder. Where is all the plunder the pirates capture from other ships?
You don't see it because they buried their treasure in chests so no one could take it from them. Want to learn how to make a pirate treasure chest craft, Mimi?