I would define "confession" as being honest with God and others about who we are, all of who we are, the good and the bad, but especially fessing up to our faults to God and fellow believers.
Confession of sins is important, because it allows God to forgive and cleanse us of the sin. In addition, in confessing, we are not only honest with God and others, but we are preventing self-deception.
If confession of sin stops, the one who doesn't confess, and their relationship to God and His church are again strained.
If we confess our sins, God can be trusted to forgive us and cleanse us from sin.
The difference between (God's)forgiveness and cleansing, is that God's forgiveness concerns Him not holding us to the verdict of sin we have committed; through the blood of Jesus, we do not have to pay the price required for sin. Cleansing refers to God helping us overcome the sin that we are in so that we don't have to go back to it, as well as washing our conscience, so we don't have to bare the burden of the guilt of sin.