Spy holes

I thought the whole purpose of a spy hole was to be able to spy on whoever's outside, without them knowing you're in.



This makes me wonder why some front doors have spy holes even when the door has glass windows? It's hardly spying when the person outside can see you clearer than you can see them...
If you had a slim rectangular body maybe it would work but i'm yet to come across a person with a rectangular head. You'd also have to walk up to the door directly central which could be difficult.

Spy holes on glass doors are a FAIL if you ask me.