Full metal jacket round is designed to wound - it is designed to go right through. It is considered humane to take your enemy out of the fight by wounding, rather than killing.
For the same reason - FMJ bullets are not permitted to be used for hunting in many areas - the object of hunting is to kill, rather than wound.
Commanders figured out later it was better to wound the enemy, rather than kill him - a dead solder is 1 person out of the fight, but a wounded soldier takes 2 or three fellows to take care of him. So, by wounding one enemy soldier, you would take several enemy out of the fight.
That's the old "conventional" wisdom, anyway.