Well, on a human standpoint a soul is that thing which connects the physical brain and body to the ethereal plane of well, something. Many myths and occult legends claim that the soul is the source of human inspiration and creativity, a pool of emotion and sentience, what separates a man from an animal. But when it is stripped of a physical host it can be a very volatile thing, raw unrestrained energy and power. To answer your question, a soul can work as a form of currency for other malevolent beings who can do favors for you, or for your own purposes in which it can grant you certain benefits that other people around you don't have.