What is the purpose of the punishment, if repentance and forgiveness have already happened?
As the definition says, perhaps the person who harmed someone did not do the reparation while alive. In other words, repair the wrongs they had inflicted on othrs.
Everything we do affects someone else.
I heard a Catholic Apologist on “Catholic Answers” explain it this way.... imagine a nail being hammered into a piece of wood. The nail represents the sin. Removing the nail represents Christ dying on the cross and removing our sins. Once the nail is removed, there is still the hole. That hole needs to be filled in/repaired.
Another way to look at it .... why put criminals in jail if Christ has already paid the price, or, why does a parent discipline/punish their child if Christ already paid the price? I could go on and on.
From New Advent:
http://www.newadvent.org/cathen/12575a.htm
Temporal punishment
That temporal punishment is due to sin, even after the sin itself has been pardoned by God, is clearly the teaching of Scripture. God indeed brought man out of his first disobedience and gave him power to govern all things (Wisdom 10:2), but still condemned him “to eat his bread in the sweat of his brow” until he returned unto dust. God forgave the incredulity of Moses and Aaron, but in punishment kept them from the “land of promise” (Numbers 20:12). The Lord took away the sin of David, but the life of the child was forfeited because David had made God’s enemies blaspheme His Holy Name (2 Samuel 12:13-14). In the New Testament as well as in the Old, almsgiving and fasting, and in general penitential acts are the real fruits of repentance (Matthew 3:8; Luke 17:3; 3:3). The whole penitential system of the Church testifies that the voluntary assumption of penitential works has always been part of true repentance and the Council of Trent (Sess. XIV, can. xi) reminds the faithful that God does not always remit the whole punishment due to sin together with the guilt. God requires satisfaction, and will punish sin, and this doctrine involves as its necessary consequence a belief that the sinner failing to do penance in this life may be punished in another world, and so not be cast off eternally from God.