"It restores the unity between the will, the intellect and the body."  What did the church do before there was a sacrament of penance?  It did not exist during the Apostolic Era,  And wasn't around when St. Augustine lived.

The Eucharist WAS the first sacrament of forgiveness.  Penance came after the persecutions of the Christians were somewhat over.  It existed for those who committed the sin of apostasy, murder, and adultery.  It was imposed on a sinner, not by any priest, but by the Christian Community itself.  And it was the Christian Community, not any priest, who chose, after sufficient public penance was performed, to accept or not---the penitent back into the Christian Community.