Mirrors don’t create light; they redirect it. If you hang a mirror on a wall that faces a blank wall, you’ll mostly just reflect that blankness. But if you place it opposite or at an angle to a window, lamp or bright area, it bounces that brightness deeper into the room.
A mirror opposite a window can double the sense of daylight and view. One opposite a lamp can spread its glow into darker corners. Thinking about what the mirror will actually reflect before you hang it makes its impact much stronger.
Random placement gives you “a mirror on a wall.” Thoughtful placement gives you more light and a sense of extra space.