I like using mirrors in portrait shots for two reasons -
First - it gives the model an opportunity to model with themselves - to create a new dynamic to their poses and choice of stance/posture/positioning.
Second - mirrors do amazing things with light. They show us things about light that we wouldn't normally see. Just 10 degrees difference in the angle that a mirror rests - in relation to the placement of the lens and the space the model is occupying - can result in a dramatic change in how the light and resulting shadows are revealed.