Along with the wooden peg leg, parrot, and oversized hat, the eyepatch was an essential piece of attire for every classic pirate. Contrary to popular belief, most pirates did not have a missing eye under that patch. Ironically, they used eyepatches as a source of sight! When entering a dark room after being in direct sunlight, it can be difficult for the eyes to adjust.
It can take up to 25 minutes for your eyes to dilate to dark areas. Pirates would frequently move above and below the deck of the ship and the patch would give them the ability to see in the dark quickly. This also gave them a higher chance to point out their enemies that would board the ship and prepare to fight instantly.
The Mythbusters attempted to understand the long-lost tale a little bit further and proved it to be plausible by testing the theory. In this day and age, people will continue to mimic the pirate eye patch as more of a fashion accessory than an effective historical product.