Do you watch MythBusters? If not, consider it, great show.
One of the recent shows talked about the Pirate eye patch. Personally I always thought it was because someone lost an eye… But there is another reason… To assist with night vision. The MythBusters showed pretty conclusively in my opinion that the eye patch helps significantly in situations where you need night vision and you are exposed to a lighted area. They showed that it can take up to 25 minutes for your eye to get the “Rods” working properly in low light situations normally, but with an eye patch it is almost immediate.
Any pilot will tell you this. When flying at night, it is extremely important to preserve your night sight because of the amount of time to recover it. Another reason why pilots use red lights in the cockpit: to preserve that all-important night sight for night flight.
I saw this episode and while they did show conclusively that night sight is preserved with the user of an eye patch, I don’t believe the Pirates and “Olde Saylors” used them for that reason.