He uses this to his own benefit, talking Cassio into pleading with Desdemona and inducing Othello to believe that Desdemona is having an affair.
Iago becomes very jealous of Othello because of his position in the Venetian Army. I feel badly for Othello because he truly believes that Iago is being a good friend and watching out for him when in fact he is doing the exact opposite!
Then Iago stabs Cassio from behind. He tells her to shut up, but she tells Othello and everyone present that Iago is to blame for the whole situation. No, not tonight… I shall not dine at home.
Iago even acts as if he is trying to calm Othello down because of all the hate he has towards Cassio after Iago shares the evidence with him in the middle of page He uses many tactics to persuade Othello that Desdemona is cheating on him with Cassio, the reason, is revenge.
This in turn leads him to get doubts about other people, for example, Desdemona; it also makes him less trustworthy of other people.
After Cassio is fired, Iago suggests to him that he ask Desdemona to intervene with Othello in his favor.