On Confession: To confess your sins to God is not to tell him anything he doesn't already know. Until you confess the, however, they are the abyss between you. When you confess them, they become the Golden Gate Bridge. (Wishful Thinking, Frederick Buechner)
Have you ever done something (or not done something) that you kept to yourself? Have you ever stopped short of confessing a wrong to someone else? That's a rhetorical question; I think we all could answer yes. What happens when you run into that person? Do you feel the gulf between you? Maybe, if it's a "small" thing, that gulf doesn't last very long as the failure to confess fades into the background of your memory, but I think we feel it.
What if the person already knows? Have you ever known what your child did, but waited for the child to tell you? You both know the sin, and there is a division between you.
Do you think that division is caused by a lack of honesty?
Could it be that we need to confess our sins to God, who already knows what they are, to rid ourselves of that feeling of dishonesty with the divine? The God we hope to have a relationship with?
I know that I can forgive people without their confessions of the wrong to me. I can forgive them, and they may not (probably won't) ever know that they have been forgiven. Because I know that, I believe that God forgives us, without our confession of sin. And yet, how do we know? How will we repair that gap between us? Confession brings us back together - and we need it, not to earn the forgiveness, but to become aware of it. Confession is not for the benefit of God; it is for our own well being, and to strengthen the relationship between the divine and us.