There are different types of demons. The jinn like Lilith require a host to become corporeal while other demons, the very powerful ones have a corporeal form. Then of course there are the insanely powerful demonic beings like Abaddon that have a corporeal form but can also possess a human usually for the sake of restoring power.

Then of course there are the senior demons (like Lilith) who need a host but can warp and alter there hosts attributes to suit themselves like changing the skin color or adding fangs and claws that sort of thing.