The primary reason why Catholic Church asks her members to confess their sins to a priest is simply because the Church has always believed that sin, however private, is a community affair. Every sin, however small, wounds the Body of Christ (the Church). In the confessional the priest is the representative of God and the Community. In the confessional the priest represents the whole Christ, the Head (Jesus) and the members (the Church).
Early Church Fathers on the Sacrament of Penance
Irenaeus speaks of making an outward confession (versus remaining silent) upon which the hope of eternal life hangs, but it is not yet clear from irenaeus just how, or to whom, confession is to be made. Is it privately, to the priest, or before the whole congregation, with the priest presiding? The one thing we can say for sure is that the sacrament is understood by irenaeus as having originated in the infant Church.
Later writers, such as Origen (241), Cyprian (251), and Aphraates (337), are clear in saying confession is to be made to a priest. (in their writings the whole process of penance is termed exomologesis, which means confession_ the confession was seen as the part of the sacrament). Cyprian writes that the forgiveness of sins can take place only “through the priests.” Ambrose says “this right is given to priests only.” Pope Leo I says absolution can be obtained only through the prayers of the priests.
Theses utterances are reminders of accepted belief in early Christian history.