Why does the Phantom of the Opera wear a mask?

In the 2004 film "The Phantom of the Opera," and in the original novel by Gaston Leroux, the Phantom's mask is used to cover a birth defect. In other film and theater adaptations, the mask has been used to cover scarring caused by fire or some other tragedy.

"The Phantom of the Opera" tells the story of a man who is so humiliated by his appearance that he lives beneath the opera house, and how he becomes obsessed with a girl who is a member of the chorus. The story has been adapted into numerous film and theatrical versions, including a 1925 silent film starring Lon Chaney and a musical by Andrew Lloyd Webber.