Why do demons and devils want souls?

[LiamEvans210]

Basically, what the title says.

In most game cosmologies it's pretty much a given that the denizens of the infernal planes are very interested in collecting the souls of mortals (whatever that means), but why?

The whole trope comes (I assume) from stereotypical christian theology/mythology, where it is largely a matter of petty spite, the Devil wanting to hurt God the only way he can, by hurting mortals. That a a little bit of misery loves company.

But in a world where demons and devils can be fought physically, there are multiple gods and they can be communicated with, and traveling to hell or the abyss or whatever to battle the forces of darkness is an option (at least for some), that reason doesn't seem like it really works.

So why? What do the extra planar powers gain by gathering souls?

I kinda get the impression that in D&D it is mainly just a means of recruitment. The souls get turned into extra planar beings and just continue to serve. That works well enough, I guess, but just seems a little.... bland? So there is no real difference between being a mortal and an angel or demon, just a different set of powers?

If I remember correctly, Wraith:The Oblivion had souls being not just the denizens of the afterlife, but also the raw materials needed to do things there, which is an idea I sort of like, very dark, but it kinda works. But not sure how well it would work in a bigger cosmology like a typical D&D-esque multiverse.

So I ask you all, what are some other options? What are some reasons why Demons and Devils (and the gods too) are interested in souls?