In the photo above it is clear that when viewed at this angle with the naked eye there is no apparent bright light source hitting the front of my 120 year old hall tree... yet in the mirror - placed just inches away - the reflection of the front of my hall tree when viewed from the angle provided by the mirror - shows bright reflections off the hand-carved wood scroll on the front. Mirrors are powerful tools that a photographer or cinematographer can use to capture an alternate but simultaneous view of their subjects. Regardless if it is a still life or a person - light and even the very manner in which a subject is perceived can be greatly altered through the reflection cast in a mirror.
I will often use mirrors in my shoots for they can add much in the way of altering how light and your subject interact. Notice that from this angle (shot by my assistant) that there are multiple shadows (being cast from the light and the light's reflection in the mirrors) behind the main subject.
Light can be bounced - redirected - altered - or focused through the use of a mirror. In this - the shot I took with my camera a moment after the photo above was shot - there are no shadows visible from this angle. Mirrors can distort light and trick the eye.
You can capture multiple facets, sides, features of your subject in a single shot.
Or you can use a reflection to help tell a story - or help focus a particular point of view.
Mirrors allow you the ability to break a visual reality by adding a totally different angle/view/reality in the same frame.
Mirrors can help you present a unique view of the world around you that otherwise might not be communicated visually.
Try shooting some photos through or using a mirror in the frame... Let me know if you think the mirror added something of value to your photo(s). I'd love to know! I personally try to shoot through or with a mirror whenever I can do so naturally (for the mirror is part of the environment I find myself working in) or if motivated... in narrative film/video/photography motivation is crucial for building a believable world - a story that supports the "suspension of disbelief" - or the ability to get lost in the narrative and forget you are watching a story. Try it sometime! :D