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?