What can be reasons of wearing sunglasses while in a company (in public)

There are people (mostly I notice celebrities do this) that (keep on) wear(ing) their shades while they are in conversation with a talk show host or when they are interviewed for a television program.
What could their reasoning to do so be?
I have learned that this is an impolite thing to do, as you should let your (conversation) partner be able to look you in the eye (and vice versa).
Could it be shyness?
Posing?
Try to hold on to an image?
Sheer impoliteness?