[Whitelaughter]

One answer from mythology is raw materials. If a soul is either a copy of the body or the original blueprint, then each soul provides a body's worth of stuff to convert into useful objects.

Frex in Norse mythology, Naglfar or Naglfari (Old Norse "nail farer") is a boat made entirely from the fingernails and toenails of the dead. Human hair could be turned into clothing, bone and keratin used to turn the clothing into armor.
Bone makes reasonable stone age weaponry.

I flipped this in Amber to have the demons providing weapons and armour to the Chaos Lords; if the Logrus power was used to acquire an item from the Black Zone, it would be torn from a demon's body - and then the demon who provided the item would enter its breeding cycle, resulting in the demons breeding for every more useful items for their lords.