The why:

Dubbing is sometimes performed to limit damage caused by injury or frostbite. Dubbing for some breeds has become a tradition and is required for some birds to meet breed-specifications.[3] Other reasons include removing combs which have become so large they prevent the bird from taking food into its mouth or making the head so heavy it sinks into the bird's chest.

Because they don't belong to the rest of the world, and neither should their breeding be continued. Jungle fowls are native to South East and South Asia only, and they live as wild birds. Domestic fowls are a fucked-up product of selective breeding.