If the spiritual father gives you a penance, accept it with joy. If the spiritual father does not give you one, then ask him to. This will be not only a send-off to you as you depart on your good path, but also a shield and protection from outside enemy attacks on your new way of life. Here is what the Patriarch of Constantinople wrote in answer to the Lutherans: "We accompany the absolution of sins with penances for many respectable reasons. First of all, so that through voluntary suffering the sinner will be freed here from onerous involuntary punishment there, in the next life, for the Lord grants mercy to nothing more than He does suffering, especially voluntary suffering. Therefore St. Gregory also says that God's love is granted for tears. Secondly, it is in order to destroy in the sinner those passionate desires of the flesh which give birth to sin, for we know that opposites cure. Thirdly, it is so that the penance would serve as a bond or bridle for the soul, and not allow it to again take up those same vices from which it is still being cleansed. Fourthly, in order to accustom it to labor and patience, for virtue is a matter of labor. Fifthly, it is so that we will see and know whether or not the penitent has truly come to hate sin.The English edition references "Christian Reading, 1842, vol. 1, p. 244 [in Russian]."
This is an illuminating passage for several reasons, first and foremost that it gives five very good reasons for the giving of penance during confession, but also because it shows that the practice is not simply a Latin borrowing or inappropriate Latinization. Finally, the first reason St. Theophan gives is illuminating regarding the Orthodox teaching on reparation for sins - the "involuntary punishment" he speaks of cannot be the permanent fires of Hell (since the sin for which the sinner would be punished is already forgiven by the Mystery of Confession), so they must be something analogous to the Latin doctrine of Purgatory.
Our holy father Theophan the Recluse, pray to God for us!