Retribution is needed to prevent vigilante justice and to cause fear to potential criminals. Remember the story of the drunk driver who killed several people in a crash [0]? He was "punished" by going to a club med like rehab facility where he was taught about the wrongs of his actions by horse back riding. Imagine if all drunk drivers were punished that way?
0: https://en.wikipedia.org/wiki/Ethan_Couch