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.
Have you ever looked at a property online but the windows are boarded up and there’s no natural light? Or gone to view a new flat only to find it’s full of a lifetime’s-worth of clutter and cardboard boxes? It’s the supernatural equivalent of that. 

Demons simply won’t want to inhabit a body if it’s cluttered up with icky internal organs and hasn’t got a nice view to the outside world. Etching the runes is an occult ‘property for rent’ sign (and a cosy corporeal body is waaay better than your typical infernal bedsit), but before any tenant will move in and start working for the roof over their head (so to speak), they want to know that the place will be properly cleaned out and opened up or they simply won’t sign the lease.

Basically your average dark sorcerer needs to be a reasonable landlord or not even the prospect of a physical bachelor pad will draw in the demons.