Author's purpose for using a mirror varies. For example Laurie Halse Anderson, in her book Speak, uses the mirror as a symbol, to show Melinda's frustrations towards herself, such as appearance.
Seeing that Melinda was rap ed, she after avoids the mirror at all times. This can be interpreted as her inability to face her feelings. She also states "she can't see herself", and that shows us that her sense of identity has been altered by the rap e.
It's safe to say that a mirror can be a symbol of physical and spiritual reflection. Hope this helps.