I think that, deep down in the heart of it, POTO is a love story that shows how superficial people can truly be.
The original Gaston Leroux version shows this by having Christine chose to leave the Phantom and go with Raoul, but after kissing him. Still, even though she has a spiritual connection to Erik, she would rather be with Raoul.
Susan Kay challenged this by having Erik and Christine have sex before he died, which added a sense of intimacy to the story.
I think the purpose is to learn to love someone for who they are underneath the skin, and not ostensibly. You know, don't judge a book by its cover. Who agrees?