Is it necessary that a Christian pray for forgiveness of sin?
We, as Christians, should acknowledge our sin, even those sins we are not cognitively aware of, NOT that we do so concerning our salvation as that is secure by covenant promise (Ephesians 1:13-14; John 10:28-30) but the Christian apologizes to our Heavenly Father for our shortcomings in order that intimacy with our Heavenly Father remains pure, uninterrupted, unaltered or undiminished by our misbehavior; this, not unlike we would apologize to a parent or dear friend that we’ve inadvertently wronged or insulted in some way.
There is no greater joy in this World and in the World to come than intimacy with our Creator through the indwelling Holy Spirit and being grieved by our sin as a Christian is a fruit or sign of the Spirit working through the conscience providing a sign of authentication, validity, that our faith is genuine and our covenant relationship secured in the Holy Spirit. Jesus said that you will know them by their works, their behavior, the way they conduct themselves (Matthew 7).
1 John 1:9 is provided for the redeemed in Christ and we seek forgiveness, we apologize, for our own good, for our own peace knowing that when we love someone we seek to honor them and not insult them, we seek intimacy and goodness; this truism is no different than our relationship with our Heavenly Father.
Truth be told, if the Father was to respond to our prayers of apology as His children, He would most likely ask “What sin?” as the Father has, by covenant promise, agreed to remember our sin and iniquities no more, forever (Hebrews 8:12); with that said, it is wise to preface our prayers to the Father with an apology for our short comings in the flesh and thank Him regularly for His mercy and love.