It’s not always possible to get to the course to play or practice so working in front of a full length mirror at home or the range enables you to check what you feel you are doing and what is actually happening are the same thing. You can use a mirror to practice many drill and to check how you align and position the body. By using a mirror you can make sure your angles are in order so that your body is geared to make a balanced swing.
Looking down the target line, you can check the quality of your alignment, and monitor the path and plane of your swing as you run through some of the key positions in the backswing: moveaway, setting of the wrists, at the top, starting down and so on.