You'll be surprised when you find out.
For years, I assumed pirates wore eye patches either to cover a missing eye (punctured by a careless cutlass) or else to protect one eye from flying splinters.
But the truth is far more exciting!
Pirates traditionally fought their battles on the high seas of the Caribbean, on enormous wooden ships.
On deck, the sunshine would be blinding, reflecting off the water.
But once a pirate swung over to the enemy ship and 'shivered their timbers', he'd often be forced to leave the blood soaked deck and take the ongoing battle into the bowels of the ship - and pirate ships were rarely well lit.
It takes between five and six minutes for the average person's eyeball to gain 'night vision' and allow them to see clearly in the darkness (which is why you get blinded when somebody flashes a torch in your eyes.)
Five or six minutes, when you're hacking and slashing for control of a pirate ship, is a very long time.
So canny pirates would leap blindly below decks and then swap their eye patch from one eye (which had no 'night vision,' since he'd been above decks in the bright sunshine) to the other (which had been covered and had adjusted to a low light level.)
Almost instantly, they'd be able to continue the fight from the bright sunshine to the murky darkness. A very clever trick.
It makes perfect sense. My father told me that he'd been told to screw one eye closed when he was on guard duty with the RAF, so if an enemy switched off the lights or tried to blind him with a torch, he'd be able to open the other eye and still have his night vision.
It's interesting to think that this little 'trick' has been on the battlefront for hundreds of years.
And that, my friends, is why a pirate wears an eye patch.