Speaking of his daughter, Brabantio tells the Duke:
We know that Iago, through Roderigo, has put Brabantio up to this line of prosecution against Othello. We know that Iago is a villain from the start, that he thinks Othello has slept with his own wife, that Othello passed over Iago to appoint Cassio his lieutenant, that Iago wants revenge.
We also know, through Iago's jibes, that Brabantio is a racist. He believes that Othello has used spells and medicines against his daughter. We know that Brabantio has had nightmares that Othello has stolen her. We know that Brabantio worries that his grandchildren will be beasts, instead of humans. Such were the European fears of miscegenation.
We know all this before Brabantio states his case. But the Duke does not. The Duke doesn't know that Brabantio has been baited by Iago through Roderigo. The Duke doesn't know that Brabantio is such a racist, although he should suspect it in scene iii. The Duke doesn't know that Desdemona has eloped of her own volition, knowing full well that her racist father would not condone such a marriage.