Like if you just captured a group of enemies and you are being charged At by another wave why let them be freed to chase you down and slaughter you minutes later
You provide wounds, like gunshots to the arm or leg, that prevent the prisoners from rejoining and fighting instantly, and subsequently need to be treated by the opposing force when they liberate said prisoners.
In fact with simple wounds like leg and foot wounds you could remove even more troops from the front than you would by killing the prisoners, as those wounded prisoners will require multiple able-bodied soldiers to evacuate them.
