Last prayer is also important.
By portraying Jesus' last prayer John is preserving an image of what mattered most to Jesus. It was not really about history in the sense of trying to reproduce what actually happened. It was more about an image and an understanding which would speak to his own and future generations. While the wording doubtless owes much to John's historical imagination, what it represents is in the deepest sense historical. It represents the central concern of the event of Jesus.