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.
I like the nail illustration. Thank you.