Blindfolds

Blindfolds can be used in various applications.

As a Sleep mask: They block out light when sleeping, especially during air travel, or for those who sleep during the day. It is believed that shutting out light allows the user to achieve a deeper level of sleep. They can also provide relief from claustrophobia for Magnetic resonance imaging (MRI) patients.

In sexual play to increase anticipation, thrill and the use of the other senses, often in BDSM play. Being blindfolded forces the wearer to rely on the sense of touch, smell, taste and sound to experience his or per partner and this is widely perceived as a thrill-seeking and sensual experience during acts of love and foreplay. 

During music, martial arts and weight lifting to encourage reliance on other senses, such as touch or hearing.

As a sensory deprivation tool in meditation (to focus attention on oneself rather than outside imagery).

A kidnapping victim, hostage, prisoner, etc. may be blindfolded to avoid identification of location(s) or people. (A common alternative to this usage is the use of a hood instead.)

Sometimes by guests who are not captured are still blindfolded or may consent to be blindfolded for the same reasons, as occurs in fiction when being led/driven to their respective hideouts.

Before sunglasses, they were worn as the primary facial cover for those who could not see. This usage, though uncommon, is still practised in modern times