Albert Barnes on Matthew 18:31

So when his fellowservants, etc. This is a mere circumstance thrown into the story for the sake of keeping, or making a consistent narrative. It cannot be intended to teach that other Christians should go and tell God What a brother had done; for God well knows all the actions of his children, and does not need us, surely, to inform him of what is done. It is abusing the Bible, and departing from the design of parables, to press every circumstance, and to endeavour to extract, from it some spiritual meaning. Our Saviour, in this parable, designed most clearly to exhibit only one great truth--the duty of forgiving our brethren, and the great evil of not forgiving a brother when he offends us. The circumstances of the parable are intended only to make the story consistent with itself, and thus to impress the general truth more fully on the mind.