Well, I'm not quite sure what you mean by "already forgiven." We confess our sins and ask for forgiveness in order to be forgiven, not the other way around. Only God forgives sins. Since Jesus is the Son of God, He said, "The Son of man has authority on earth to forgive sins" and exercises this divine power: "Your sins are forgiven." Further, by virtue of his divine authority he gives this power to the Church to exercise in his name.
Christ has willed that in its prayer and life and action, His whole Church should be the sign and instrument of the forgiveness and reconciliation that He acquired for us at the price of His blood. But He entrusted the exercise of the power of absolution to the apostolic ministry which He charged with the "ministry of reconciliation." The apostle is sent out "on behalf of Christ" with "God making his appeal" through him and pleading: "Be reconciled to God."
2 Corinthians 5:18-20 - Now all these things are from God, who reconciled us to Himself through Christ and gave us the ministry of reconciliation, namely, that God was in Christ reconciling the world to Himself, not counting their trespasses against them, and He has committed to us the word of reconciliation. Therefore, we are ambassadors for Christ, as though God were making an appeal through us; we beg you on behalf of Christ, be reconciled to God.
Acts 19:17-18 - Fear fell upon all, both Jews and Greeks, and the name of the Lord Jesus was being magnified. Many also of those who had believed kept coming, confessing and disclosing their practices.
James 5:16 - Therefore, confess your sins to one another, and pray for one another so that you may be healed.
John 20:19-23 Jesus said to his disciples, "Peace be with you! As the Father has sent me, I am sending you." And with that he breathed on them and said, "Receive the Holy Spirit. If you forgive anyone his sins, they are forgiven; if you do not forgive them, they are not forgiven."
Matthew 3:6 Confessing their sins, they were baptized by him in the Jordan River.
Confession does not involve confessing sins to man instead of God. A person's sins are forgiven when they repent, but Confession restores back a person's relationship with God and His Church. Confession is the act of confessing sins to God in the presence of one of God's ambassadors (2 Corinthians 5:20), the priest.
2 Corinthians 2:10 What I forgave, if I have forgiven anything, it was for your sakes in the person of Christ.
Jesus said to His Apostles, "If you forgive anyone his sins, they are forgiven; if you do not forgive them, they are not forgiven." When the priest says, "Your sins are forgiven (Mark 2)," he is reassuring God's forgiveness in a verbal way, the same way Jesus assured people of forgiveness. (Luke 7:48 Then Jesus said to her, "Your sins are forgiven.") After the confession of sins, the priest tells the confessor to pray for forgiveness. This is called the Act of Contrition, and is usually similar to the following prayer: "My God, I am sorry for my sins with all my heart. In choosing to do wrong and failing to do good, I have sinned against you whom I should love above all things. I firmly intend, with your help, to sin no more, and to avoid whatever leads me to sin. Our Savior Jesus Christ suffered and died for us. In his name, my God, have mercy."
We need confession so we can unburden ourselves, so we can get the poison of sin out of ourselves. It helps us to examine our daily lives regularly and to make specific resolutions to avoid near occasions of sin as well as to keep from sin itself. Contrary to what many people believe, Confession is not a means of "checking in" so a person can go right back in the world with the intention of sinning. Confession involves resolve to not sin again, with Christ's help. The priest is bound to never discuss anything that is confessed with anyone, not even to another priest.
Eric