You're probably familiar with the iconic image of a pirate:
You probably assumed, as I assumed, that the eye patch was there to cover up a missing eye.
But! It turns out that pirates wore eyepatches to keep one eye adapted to the dark so when they went under deck they could switch the patch to the other eye and be able to see right away.
This was especially helpful during battles, where they might find themselves fighting in the dimly lit belly of the ship.
So there you have it! Pirates wore eye patches for dark-adjustment. Not because they only had one eye.
Source: The Wall Street Journal
H/T: Frank Stags