The Alter Rebbe explains: A mirror has two purposes. First, when one looks at a mirror, one sees his own self. Second, one also sees that which is behind him.

This is the ultimate goal in one’s life. In man’s spiritual work, he needs to see not only his present situation as it looks now, but also that which is behind him, namely yesterday, and the day before yesterday, lest he need to correct something.

Sichos Kodesh, 25 Iyar, 5723