Limited resources are always annoying due to their nature of being limited. Within games, it can always be a fun challenge to somehow gain an infinite quantity of a seemingly finite supply of a certain resource.
One resource that is particularly annoying to deal with is souls. They're a magnificent source of power, but that's also why their production costs are astronomical. A cheaper way to create new human soul is actually by converting a soul of a different species. This is one of the reasons why humans have hunted so many species into extinction.
Only by freeing up souls can the the human population count be increased further. Of course this is nowhere near a 1 to 1 transfer rate.