Ka -- that is, souls -- are used as currency in the Underworld.
Back in more ancient times, it was a simple barter system; you gave someone some Ka, and they gave you goods and services in return. It worked reasonably well, but Ka are tricky to extract from the dead, and a pain to store and transport. (They are basically ghosts, after all.)
Eventually, the jackal-headed minions of the god of Death hit on the idea of operating a Ka storage facility -- essentially, they invented currency and banking simultaneously. They harvest the Ka and give you gold coins in exchange, which are accepted as legal tender by basically everyone. The jackal demons make their profit by giving coins for real Ka at a 1-for-1 rate, when the face value of the coins is actually only .9 of a Ka. Everyone accepts this cut as the price of extreme convenience, and the jackal demons have become immensely rich.
The fact that jackal-headed death demons took to banking like a fish to water is something that should surprise exactly nobody.
Cannot be traded or discarded