When we sin we affect not only ourselves but, in an even more significant way, we affect others. Every time we sin there is a negative ripple effect. The consequences of our sin go out to affect those around us and well beyond us. Even our most private thoughts that we never speak out loud affect others since our inmost thoughts determine how we interact with others and live in the world. Because our sins are an offense against the Body of Christ it is fitting that we come to the Body of Christ to seek reparation and to be reconciled. The priest has been given the authority to "bind and loose" (Matthew 18:18) on behalf of the offenses against the Body of Christ.
Many argue, "Can I not ask forgiveness from God in the quiet and isolation of my room and will God not forgive me?" The answer is, yes, God can and might forgive you, but there is a hidden premise in that question that is troubling. The argument is, "I can do it. I can achieve reconciliation from God on my own. I don't need anybody else." The Sacraments intend to help us to experience our individual powerlessness and limitations. They remind us that nobody ever has come to know, understand, or encounter God on his or her own and apart from anyone else or apart from the Church. So, really, the answer to that question is, "No, you cannot be forgiven, made right with God, and overcome sin on your own. Christ has led you to Him only through many, many others. He blesses you through gifts He has given to others to be passed on and shared with you."
Confession, like all of the Sacraments, intends to give us a sensory encounter of Jesus in person and in the flesh. Christ was made flesh and dwelt among us (John 1:14). I cannot tell you how often people confide to me that they have trouble believing that God has forgiven them. Yet I also have experienced, in my own life and in the lives of others, what a difference it makes to hear the words of forgiveness spoken out loud to our face and/or directly into our ears, "I absolve you of your sins!" People let out great sighs of the relief or cry tears or utter spontaneous words of praise when those words are spoken. On our own, without the aid of the Sacrament, we tend more to doubt God's forgiveness. Or we might not even recognize God's words or expression of grace. It makes a difference to hear those words clearly and distinctly – and greater assurance in God’s forgiveness comes to us when we do.
Keep asking questions and learning about our very profound faith!