It's not for looking through the walls, though. It's so you can look at the sky without going outside. If you don't believe me you can try it yourself. It looks far, far away, so if you tried to just look in the next room or something it would just be all out of focus.

Plus I think it somehow knows not to look in bathrooms or anything like that. Because Rhys-M one of the blokes thought it'd be funny to look in the Prefects' bathroom but it just only showed him corridors and classrooms and the lake and such.