What are souls good for?

I'm playing Life & Death as spoiler free as possible, and two of my ghost sims have acquired an object called "soul". One bought it from Waylon, the other received it from a quest. Can someone explain in the least spoilery way possible what this object will be used for later? 

(I need to know because I'm also blogging my gameplay and souls being something you can "have" and trade instead of being what you literally ARE messes with my lore quite a lot. So if this soul object is just a crafting ingredient, I'd simply call it something else in my story. A coalesced chunk of memories or emotions or somesuch.)