The act of confession is not for the sake of gaining God's forgiveness, it's for the sake of gaining our own. And of remembering the importance of forgiving others, as we also need to be forgiven, by them. Without this, sin becomes nonchalant, and forgiveness becomes assumed, but not actualized.
Ever notice that it's the Christians who think they are forgiven forever and automatically, no matter what they do, that tend to judge and condemn others most often and easily? And hold grudges most emphatically? I think it's because they don't believe they have to practice the gift of forgiveness. They don't have to bother recognizing their own sins because they figure they're automatically erased, anyway. They don't need the forgiveness of others, they think, because they figure God has already forgiven them. And since they don't need the forgiveness of others, they don't need to bother forgiving others, either.
When we take forgiveness for granted, as many Christians do, we tend to forget about the spiritual importance of it. While the act of having to confess our sins keeps us aware of our constant need of the gift of forgiveness, and of others constant need of it, too. It keeps us participating in the gift, and in the healing that comes with it.