There are many different names for the sacrament of penance, including reconciliation, confession, and forgiveness, which all give a clue as to why Catholics go to a priest to confess their sins.
A priest acts In Persona Christi, or in the Person of Christ. It is not the priest who absolves the penitent from sin; rather, it is Jesus Christ through the sacramental ministry of the priesthood. The words of absolution, or forgiveness, hint at this, since it is said in the first person.
If the “I” referred to the priest, then the sacrament would have no effect. Rather, the first person is Christ. “I absolve you from your sins, in the name of the Father and of the Son and of the Holy Spirit.” Second, sacraments are encounters with divinity. Sacraments are the chief way in which God communicates grace, His saving life within us. The sacrament of penance is an encounter with the healing Jesus. It is through the words of absolution that the merits of the blood of Christ are poured over the penitent. Thus, we are healed and forgiven in the blood of Christ.
Third, every sin, no matter how personal, is an offense to Almighty God. Like Adam and Eve, filled with pride through our conscious and free will, we decide what is best for us rather than allowing God to decide for us. Classically, sin is defined as a turning away from God towards a lesser thing.
Since all sin ultimately involves pride, the sacrament of penance is a true act of humility which counterattacks pride. It is a humbling experience to confess one’s deepest faults to another person, even though that person, a priest, is acting in the name of Christ. By this act of humility, we express to Almighty God how truly sorry we are that we offended Him. The penance that is given by the priest helps us to work out our offense and practice the virtue of humility. While it never erases the insult sin commits against Almighty God, it certainly makes our guilt less.
Fourth, it is a sacrament of reconciliation. Sin not only separates us from God, but it also disrupts our relationship in the mystical body, the Church. Mortal sin cuts us off from that relationship. Therefore, the sacrament of reconciliation not only restores our relationship with God, but also restores us to the Church.
True forgiveness begins outside the confessional. Our attitude of true contrition and firm purpose of amendment must antecede confession. Yet our Lord instituted this sacrament as the way to communicate His forgiveness.