Many people find direct sunlight too bright to be comfortable, especially when reading from paper on which the sun directly shines. Also, with the rise of the atmosphere's damaged ozone layer, it has been recommended to the public to wear these kind of glasses on sunny days to protect the eyes from ultraviolet radiation. They have also been associated with film actors since the lighting involved in production is typically strong and uncomfortable to the naked eye.
Corrective lenses can be darkened to serve the same purpose, or secondary clip-on dark lenses can be placed in front of the regular lenses.
People with severe visual impairment, such as the blind, often wear sunglasses so they do not make others uncomfortable with the fact that they cannot make eye contact with them.
James Ayscough began experimenting with tinted lenses in spectacles in the mid-18th century. These were not "sunglasses" as such; Ayscough believed blue- or green-tinted glass could correct for specific vision impairments. Protection from the sun's rays was not a concern of his.