I agree with the above post. Because Iago understands Othello's inner vulnerabilities and personal weaknesses, he is able to use them against Othello when building doubt and suspicion against Desdemona. Iago knows that the handkerchief that Othello gave to Desdemona is symbolic of his past life and incredibly important to him. Iago asks Emilia to take this precious token to fire a rage in Othello.
Iago finds a way to turn Othello's incredible devotion to Desdemona against him by building a suspicion within him that he is cheating on her. He also finds cracks Othello's own confidence that Desdemona loves him as he points out that, after all, Othello is a man of war and a Moor and not someone that a princess type like Desdemona would really go for.
He does this over time by insinuating that there are problems, and that there is something going on between Cassio and Desdemona, making it so that the very innocent friendship between them looks like something more, eventually driving Othello into a rage. This, along with the absence of a purpose for Othello in terms of a war to fight, combine to overcome his reason and drive him to madness.