Player Characters have a key advantage over enemy creatures: death saves. Death saves allow PCs to (under most circumstances) return from the dead - they get knocked down to 0 HP and have 3 chances to come back to life. The living enemies mechanism gives the same general consideration to enemy creatures.
Simply put, this rule assumes that enemies who are dropped down to 0 HP are not killed outright. The creatures are instead rendered Unconscious unless they take additional damage, or are resuscitated by another creature.
Characters who wish to slay enemy creatures must use an additional attack action after the creature has been downed in order to kill it. This adds narrative and moral weight to combat by making a distinction between the act of defeating an enemy versus taking effort to murder them. It also "murder hoboism" less incidental and more deliberate.
There are, however, circumstances that lead to the immediate death of a creature. If the last attack on a creature is a critical hit, or if the amount of damage of the final attack is at least twice their total HP, they die.
If you combine this rule with something like 'Lingering Injuries' and you could have narrative situations where a downed enemy can survive, and re-emerge with battle scars from their last encounter with the party. Whether they are back for revenge or grateful for the mercy is entirely up to the DM...