As a child of God, we often anticipate this petition. We are daily in our sins and need the forgiveness of them. There is no perfection in this life and sin attaches itself to everything we do. This petition is not one for justification. It is one we pray as we live in sanctification. We pray this petition that we may have peace of mind as we live here below - dealing daily with our sin. Why do we need to pray this petition? Because we are sinners ans as we walk in our sin, we need the knowledge that we are forgiven. How wonderful!