What was the author's purpose for The Phantom of the Opera?

The author uses the phantom as a symbol to represent the differences between what is only appearance and what is, indeed, reality. The fact that he wears a the mask is a cue to his need to be seen another way, rather than for who he is. Because of the mask, Christine can believe him to be a poor, misunderstood artist. When she sees him without a mask she is applauded and can no longer think of him as she did before. His appearance is not only all that the author wants the reader to notice. The reader should notice that he is hiding his true self from the world. He  has many gifts but they are never acknowledged for that which he is truly talented. He is the phantom because nobody is quite sure even if he exists.

The author's purpose is to show readers that hiding behind a mask has a dual function: 1.) to present to the world what they or society wishes to see and 2.) to hide from the world what one wishes they or society would never see. Either way, one cannot be recognized for one's true self in the guise of a mask.