I would suggest that the answer to this question lies in the effects & graces of the sacraments & what they confer; in Baptism, one receives forgiveness of all sins, becomes “a new creature,” becomes engrafted into the Church/Body of Christ, forms a sacramental bond with all the baptized, & the receive an indelible mark on their soul.
With the sacrament of Penance, those who have been baptized & have received all the graces of the sacrament of Baptism must be restored to God’s grace and intimate friendship with God. Reconciliation with God is thus the purpose and effect of this sacrament. The penitent needs to be reconciled us with the Church & all of creation because the sin the penitent has committed damages and/or even ruptures his/her communion with God & the Church that he/her first received in Baptism. The sacrament of Penance repairs or restores this.
With sin, there is what we call double consequence: separation from God & attachment to that which is not God. Mortal sin deprives us of communion with God and therefore makes us incapable of eternal life, but even venial sins themselves entail harmful attachments (i.e., the “temporal punishment” of sin), which must be purified either during our temporal life on earth, or in Purgatory. This purification is necessary because of the very nature of sin.
The forgiveness of sin & restoration of communion with God that takes place in the sacrament of Penance truly involves the absolution of the eternal punishment of sin, but temporal punishment of sin remains, which acts as a sort of grace for us. It calls us to works of mercy, charity, prayer, & various acts of penance, to put off completely the “old man” that we once were before Baptism & to put on the “new man” that Baptism made us for.
Does that help?