The Catholic Church (and all apostolic Churches, for that matter) require confession of sins to a priest. Most Protestant churches, who can't trace their history back to the Apostles, believe confessing your sins directly to God is sufficient. While repantance for sins is a requirement, the Catholic Church has always required the confession of sins to a priest for reintegration into the Christian Community. The Catechism teaches us:
1444 In imparting to his apostles his own power to forgive sins the Lord also gives them the authority to reconcile sinners with the Church. This ecclesial dimension of their task is expressed most notably in Christ's solemn words to Simon Peter: "I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven."45 "The office of binding and loosing which was given to Peter was also assigned to the college of the apostles united to its head."
1445 The words bind and loose mean: whomever you exclude from your communion, will be excluded from communion with God; whomever you receive anew into your communion, God will welcome back into his. Reconciliation with the Church is inseparable from reconciliation with God.
Christ and his Church are one, so to sin against Christ is to sin against His Church, so to be reconciled with Him you must also be reconciled with His Church.
So why can't we be reconciled with His Church directly through Him? After all, even the Catholic teaching is that the priest is merely there as a standin and that it's throgh Christ we are reconciled, rather than through the priest. An article I was reading in the June 6th issue of Our Sunday Visitor
I think gives a very strong reason for this. It's for our benefit. This quote is from their "In Focus" section on "The Effects of War" in an article specificially dealing with the effects of killing on a soldier:
Father [John] Barkemeyer [an Army chaplain] witnessed how "tremendously powerful" the role of Catholic faith, especially the sacrament of reconciliation, is in the lives of the troops.
"'I grant you pardon and peace, and I absolve you of your sins.' That's hugely powerful for guys. Where else can you hear the words that you need to be able to hear so desperately?" he said.
The full formula of absolution
used in the sacrament of Confession is:
God, the Father of mercies,
through the death and the resurrection of his Son
has reconciled the world to himself
and sent the Holy Spirit among us
for the forgiveness of sins;
through the ministry of the Church
may God give you pardon and peace,
and I absolve you from your sins in the name of the Father, and of the Son and of the Holy Spirit.
God knows sometimes, in our weak and fallen state, we need more than just the belief He has forgiven us. So, through his ministers, He tells us in a voice we can hear.
Here's a story I heard that I think illustrates the point:
A young girl is in bed, being scared by a thunderstorm outside and asks to have someone come in her room. Her mother says, "God's in there with you." The daughter says back, "I need someone with skin."
We're the same way. We know God will forgive us anything if we're truly sorry, but we still need to hear it sometimes from someone with skin.