Souls are important to deities (gods, goddesses, devils, demons and other powers) as they help define the deity's power level - the more worshippers (A worshipper is just a soul that hasn't died yet) a Deity has, and the more souls there are in their Realm, the more powerful they are. The Greatest Deities (Demons, Devils etc) have access to huge numbers of souls to help power divinity, and they always want more, that is why churches are always pushing for expansion and new followers. It is how Quasi Deities become gods and how lesser gods become more powerful. And, in the past, that has caused trouble - Chaotic and Evildeities were