The eye patch is a piece of opaque material, usually made from some kind of fabric, that is kept in front of the eye and held in place with a headband or tape. It is meant to shield the eye from light or other irritations. Eye patches are often prescribed as a protective cover to people recovering from eye surgery. Non-surgical medical use occurs in occlusion therapy for conditions that could benefit from monocular vision such as amblyopia or diplopia.
Before David Bowie made it a fashion statement and before Moshe Dayan added an extra level of badassery to it, the eye patch was an integral part of the swashbuckling getup of the pirate, to the point at which popular culture tends to think of eye patches and pirates as going hand-in-hand like bangers and mash. The patch, if a pirate had one, was hardly decorative, it being quite plausible that a pirate really had lost an eye. Even though pirates dominate the eye patch paradigm, trauma and infection used to claim many more eyeballs than they do these days so eye patches would have been relatively common, on and off the high seas, during the golden age of piracy.
Actually, the eye patch could have been used by sailors in general. Lighting conditions belowdecks were dim at best so sailors who had to go in and out of darkness may have used an eye patch to keep one eye accustomed to darkness. This is an appealing theory but I really don't see how some of the dangerous jobs on board a sailing ship would not have become significantly more dangerous without binocular vision, nor have I seen any mention of eye patches in conjunction with steamships, which were probably just as dark inside before electrification.
The same story about light and darkness goes for pilots and is better documented. SAC pilots in the earlier part of the Cold War were issued eye patches. While this did little to improve their depth perception, the idea was to protect at least one eye from nuclear flash blindness were a bomb (as in the bomb that they just dropped) to go off close to home since it's also pretty well documented that blind pilots and their cargo tend not to get very far. Bombers were later fitted with cameras and reflective curtains and in 1980 some fairly sophisticated goggles were devised. Hi-tech eye patches if you like.
It's pretty uncommon for someone to need more than one eye patch. I suppose that rules and techniques designed for total blindness apply to getting around with a pair of them. A double eye patch that's pretty common and available to business class travellers and to long-haul plebeians is a sleep mask. Since its purpose is to block all light, it tends to be of little use to ambulatory humans and is best used in the horizontal position. The ubiquity of this device suggests that the air travel industry still thinks light is more of a problem than noise. But then we do have a preoccupation with the sense of vision. Eye patches can remind those of us with a pair of working eyeballs not to take them entirely for granted.