In fairy tales, monsters exist to be a manifestation of something that we need to understand, not only a problem we need to overcome, but also they need to represent, much like angels represent the beautiful, pure, eternal side of the human spirit, monsters need to represent a more tangible, more mortal side of being human: aging, decay, darkness and so forth.
And I believe that monsters originally, when we were cavemen and you know, sitting around a fire, we needed to explain the birth of the sun and the death of the moon and the phases of the moon and rain and thunder. And we invented creatures that made sense of the world: a serpent that ate the sun, a creature that ate the moon, a man in the moon living there, things like that.
And as we became more and more sophisticated and created sort of a social structure, the real enigmas started not to be outside. The rain and the thunder were logical now. But the real enigmas became social. All those impulses that we were repressing: cannibalism, murder, these things needed an explanation. The sex drive, the need to hunt, the need to kill, these things then became personified in monsters. Werewolves, vampires, ogres, this and that. I feel that monsters are here in our world to help us understand it. They are an essential part of a fable.
Guillermo del Toro.