Why would someone wear sunglasses ? Day and night . Even indoors.?

some people may have light sensitivity. even store lights can hurt their eyes and give them headaches (store lights tend to be worse than sunlight in that regard). some people may also be trying to hide their eyes because they're high and don't want others to know. and some people may be blind and not want to freak out people with their lifeless looking eyes staring at nothing

I wear my sunglasses at night
So I can so I can
Watch you weave then breathe your story lines
And I wear my sunglasses at night
So I can so I can
Keep track of the visions in my eyes

Because he wants to hide ... if he has no eye disease

To intimidate others. This goes along with the previous point: Because sunglasses hide an individual’s face, they can attempt to express power over others by hiding their emotional vulnerabilities.
https://www.rachelallan.com/prom-dresses

because they can

They are blind

They wear them because they think they are cool.

They may have eye issues

They may be blind