Dark glasses, as an alternative form of spelling sunglasses. I can wear them "at night, so i can so i can
see the light that's right before my eyes", as sung by Corey Hart a long time ago.
Dark glasses work also as a symbol of invulnerability. The Men in Black wore them. Neo (Matrix) wore them. Morpheus and Trinity also did. Stingray wore them. The Blues Brothers also... hey wait. Well, they work as symbol of invulnerability, since the one who is wearing them can stare at anyone without remorse. Actually, i have a hard time staring at anyone right in the eyes. Don't do it in the subway, it might be interpreted as a lack of etiquette. A pair of dark glasses might come handy then.
Dark glasses may become useful as eye protection (you don't want to stare at the light if you suffer from glaucoma); for concealing a physical defect as estrabism or red eye after the consumption of hemp; or to hide supernatural powers, as does Cyclops in X-men. Of course i won't ask why a couple of sunglasses are able to stop the power of rays coming out of your eyes, powerful enough to blow a ceiling or a building if left unattended.
Dark glasses might be used also to look sexy, chic, or simply drop dead gorgeous.
By the way, the french political/countercultural movement called "Les Zazous" also wore dark glasses and long hair, trying to appear unaffected by the german occupation during WWII. And that brings me again to the Pet Shop Boys and Chris Lowe, who sang about the Zazous in their song In The Night; Chris was obviously wearing his dark glasses.