Objection 1. It would seem that confession nowise delivers from punishment. For sin deserves no punishment but what is either eternal or temporal. Now eternal punishment is remitted by contrition, and temporal punishment by satisfaction. Therefore nothing of the punishment is remitted by confession.
Objection 2. Further, "the will is taken for the deed" [Cf. Can. Magna Pietas, De Poenit., Dist. i], as stated in the text (Sent. iv, D, 17). Now he that is contrite has the intention to confess. wherefore his intention avails him as though he had already confessed, and so the confession which he makes afterwards remits no part of the punishment.
On the contrary, Confession is a penal work. But all penal works expiate the punishment due to sin. Therefore confession does also.
I answer that, Confession together with absolution has the power to deliver from punishment, for two reasons. First, from the power of absolution itself: and thus the very desire of absolution delivers a man from eternal punishment, as also from the guilt. Now this punishment is one of condemnation and total banishment: and when a man is delivered therefrom he still remains bound to a temporal punishment, in so far as punishment is a cleansing and perfecting remedy; and so this punishment remains to be suffered in Purgatory by those who also have been delivered from the punishment of hell. Which temporal punishment is beyond the powers of the penitent dwelling in this world, but is so far diminished by the power of the keys, that it is within the ability of the penitent, and he is able, by making satisfaction, to cleanse himself in this life. Secondly, confession diminishes the punishment in virtue of the very nature of the act of the one who confesses, for this act has the punishment of shame attached to it, so that the oftener one confesses the same sins, the more is the punishment diminished.
This suffices for the Reply to the First Objection.
Reply to Objection 2. The will is not taken for the deed, if this is done by another, as in the case of Baptism: for the will to receive Baptism is not worth as much as the reception of Baptism. But a man's will is taken for the deed, when the latter is something done by him, entirely. Again, this is true of the essential reward, but not of the removal of punishment and the like, which come under the head of accidental and secondary reward. Consequently one who has confessed and received absolution will be less punished in Purgatory than one who has gone no further than contrition.
Source: Summa Theologica (New Advent)