Demons are incorporate, malicious spirits from the depths of hell. They require a human form in order to pass into the mortal realm. A dark sorcerer can sacrifice an individual in order to trap a demon in the victims body, enslaving the spirit to their will. This is a risky process that can come back to harm the user if the demon isn't bound correctly. 

Ancient runes and sigils must be carved into a living victim's skin. This will bind the demon to the body, trapping it and subjecting it to the sorcerer's will. Then the victim is killed through a dark ritual. The organs are removed from the body, as well as the bones, cartilage, etc, until only the skin is left. It is then placed over a ritual circle, while the sorcerer chants an ancient incantation that opens the gates to the netherworld. The circle provides the passageway, splitting the ground and allowing the demon to inhabit the skin of the victim. The runes keep the demon in the skin of the victim, trapping it similar to a prison.

Since demons are just bodiless spirits, why would a victim's insides have to be removed in order for the demon to possess them?
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> Since demons are just bodiless spirits, why would a victim's insides have to be removed in order for the demon to possess them?
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I don't know mate, that's non traditional lore and totally on you. Which sort of makes this question "opinion based" and closeable. 

But if you are asking for a mythologically justifiable reason that could be tied to this then perhaps:

1) **(logically) preservation-** skin aka leather lasts a whole lot longer if removed from more decomposable flesh. Afterall you want your demon slave sticking around for a good long time. 

2) The Egyptians believed **the heart** was the seat of the soul, since the soul prevents possession then removing the heart is critical. 

3) **Temptation** - Physical appearance leads to raw impure carnal sexual attraction which leads to sin. By this logic the skin could be the only organ inherently unholy enough to support the demon. It also justifies self flagellation as a means of penance.
While demons ligation to the physical plane is much more tenuous than our own they are still corporeal beings. They can move through air and earth alike but density obscures their conveyance. To realize greater advantage a demon must exist in the void of a vessel; unhindered by blood and bone. For it is the boundaries between thick and thin that allows demons to gain the most purchase on the physical realm. 

The older the boundary the stronger the interaction. This is why a demon can only escape through freshly broken earth or an adult is a better surrogate than a child.