There is no need for penance. There’s no precedent for it in Scripture. Find some verses and show me where we have to perform certain deeds or acts or prayers, or anything else assigned by a priest, in order to receive forgiveness from God.
Its called "expiation", a means of atonement.
"Therefore he had to be made like his brethren in every respect, so that he might become a merciful and faithful high priest in the service of God, to make expiation for the sins of the people." - Hebrews 2:17