1: If it's a bright day.
2: If you have been given a black eye and want to conceal it.
3: You are making a (not-very-good) attempt to conceal your identity.
4: You are a complete tit who thinks wearing sunglasses makes you look like a gangster or a pop star or something.
"Er, no. He's actually an alien with red eyes and doesn't want anyone to see them".
Celebrities use them to diguise themselves as normal people. Normal people use them to digsuise themselves as celebrities.
Used in sandwiches eaten by the Inuit.
Also the surname of the ever-smiling Peruvian soul singer of the seventies, Juan Carlos Sunglasses.