Throughout the play, Iago is thought to be honest and a good advisor. We often see " honest Iago ", or in Othello's words " this fellow's of exceeding honesty ". None of the characters in the play knows what Iago is really up to, and they blindly trust him and accept his advice.
Thanks to his reputation, he manipulates Othello's feelings and reasoning, destroys Cassio's reputation, and also manages to get Othello to kill Desdemona.
Iago says it himself when he is talking to the audience " when devils will the blackest sins put on, they do suggest at first with heavenly shows ".
Without his reputation as an honest man, Iago would have never been able to do any of those things.
Othello's reputation as a serious and just military general plays a big role in this play. Without this reputation, he would have probably offered Desdemona a chance to explain what happened, and would have probably made the story have a happy ending. Because this is a tragedy, we need Othello to feel pity and fear for the hero, as he does not know what kind of a trap he is in.
He values his reputation a lot, and fearing that Desdemona has made him a cuckold proves that. He is so obsessed with keeping his reputation, that he is even ready to kill his own wife for it. This makes him so blind that he gets entangled in Iago's web of deceit, just like a helpless fish in a net.
The reputation of Cassio changes throughout the play. He turns from a very honorable and respectable lieutenant of Othello, to losing his lieutenancy and being considered an irresponsible alcoholic. " Reputation, reputation, reputation! O, I have lost my reputation! I have lost the immortal part of myself, and