Why do demons and devils want souls?

[Anaraxes]

Originally Posted by Turhan's Bey Company

[*]They're a point of status competition in the underworld. Souls are themselves without practical use, but acquiring them is prestigious.

It's possible that demons just want prestige for its own sake; they're egotistical. But another variant is that prestige itself is useful. It could just be the straight-up fuel for magical power. Or it might give a demon more sway with other demons -- think politicians -- or might let them acquire more "currency" of whatever form -- think luxury brands charging more simply because they're more prestigious. This makes souls indirectly valuable for a number of reasons suggested upthread. The souls per se might have no value, but having them leads to prestige, which in its turn is valuable.

If souls are tradeable between demons, then they become essentially currency. So it might be more interesting to decide that souls can't be swapped between demons. They have to personally earn each and every one they keep on their string. So at least some of the prestige comes from the metaphysical proof that a demon has been putting in the work, outsmarting mortals to get their souls. Other demons might want to learn from the masters to get better at reaping their own string of souls, or serve just for some reflected glory or gang identity.