A well-placed mirror can perform magic in a room. 

It can:

CHANGE THE ARCHITECTURE

By placing a mirror opposite a window, it creates the impression that it too is a window on an otherwise blank wall.   Hang a long mirror horizontally down a narrow hallway, and it widens the space and gives you a peek at rooms at the other end.  Add one large mirror or even a series of same-sized mirrors on one wall of a tiny space, and it magically grows in size while preventing claustrophia. :)  A large leaning floor mirror can create a focal point in a room where there is none. 

ADD MUCH NEEDED LIGHT

Again, reflecting a window can double the sense of light.  Also, a lamp or 2, or sconces that reflect in a mirror make a beautiful impression.

BUT...

Avoid placing mirrors where they are reflecting a bad view, or a blank wall.

Happy mirroring.  Lisa