why do blind people wear sun glasses instead of taking them off?

Answer 1

First of all, one assumes that being blind is like a seeing person having their eyes closed. In a high percentage of cases there can be some vision or sensitivity to light.
The second issue is that blindness or other severe vision problems often have a physical manifestations which may be visible in the eyes to others. This can be rather unappealing and sunglasses can hide the defect.

Answer 2

Some blind people cannot control eye movement, so they cover them up. Also, some do not blink and their eyes (because they don't need to) and can get a crusty substance around the edges of the eye.