To protect their eyes  from sun's rays of course!

One common reason that people believe that blind people wear them is for cosmetic reasons, such as to hide scars or hide their overall disability. But the bottom line is that blind people wear them more for medical reasons, than they do for cosmetic reasons.

A person with a visual impairment is generally not completely blind in the sense that, they still do see some light and fuzzy images. A blind person can also experience a phenomenon called nystagmus, which describes a condition where there are involuntary movements of the eye, generally a fluttering motion. This type of eye movement can be very disturbing to people who witness it, so blind people hide it by wearing dark glasses.

Recent studies have also linked light sensitive pathways in the brain that help perceive images and even though blind people are not able to perceive clear images, these receptors are still active causing severe migraine headaches if they are exposed to light. So blind people wear dark glasses, to avoid this from happening.