The idea behind this question is that it seems incoherent to ask God to do something he has already done. Isn’t this theologically confused, and also pastorally dangerous: rather than walking in the freedom of grace, don’t we then bind ourselves to repeatedly “working” for our salvation through repentance over sins that were already dealt with?
In terms of answering the question, let’s consider the Lord’s Prayer. It was given as a model for our regular prayer, and this model includes asking God for forgiveness (Luke 11:4).
This is surely because seeking forgiveness is a natural extension of confessing our sins, which 1 John 1:9 indicates we ought to do; indeed, it is how we claim the promise of forgiveness.
This in turn indicates the confusion inherent in the question. While we colloquially may speak of our sins being dealt with on the cross, we were not justified on the cross. We are justified by faith. The cross is the grounds of our forgiveness; it is the penalty paid that God can look to as if it were our own. But we have to ask for that to happen.
When we sin and we do not ask forgiveness, we are implicitly trampling the cross underfoot, because we are adopting a way of life that refuses to ask to be identified in Jesus and his work. Faith is not a one-time act, nor a mere belief; it is a continual effort of loyalty and trust (James 2:14–17). So asking for forgiveness is simply a recommitment to our initial profession of faith; it is our part in restoring the relationship with God that our sin damaged, and reclaiming the promise that we can be made right with him.