“If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness.” 1 John 1:9

If God has forgiven us forgiven our since because of Christ’s death, why must we confess our sins? In admitting our sins and receiving Christ’s cleansing, we are: (1) agreeing with God that our sin truly is sin and that we are willing to turn from it, (2) ensuring that we don’t conceal our sins from him and consequently from ourselves, and (3) recognizing our tendency to sin and relying on his power to overcome it

(Taken from NIV Life Application Study Bible)