The pirate eyepatch: an explanation better than the evidence.
The popular theory: pirates covered one eye so it could see instantly in the dark below deck. Physiologically plausible – an eye needs up to 25 minutes to adapt to darkness. But there is no historical evidence for it. The eyepatch as a pirate emblem comes from novels and films of the 19th and 20th centuries.
Useful: A convincing explanation is not proof that it actually happened.
Source: en.wikipedia.org