I’m doing a bit but it’s because if you want your own captured soldiers to not be killed or mistreated, you have to not kill or mistreat the captives you take. They’re also useful leverage in negotiation: “we’ll return 500 PoWs if you’ll honor such and such ceasefire” sort of thing.

Also, if you take prisoners your enemy may be more willing to surrender if their position is lost. If they know they’re all going to die no matter what, they’re going to make sure to hurt you as badly as they can before they go.

Taking no prisoners makes you worse at warfare.