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He sees her as Desdemona's friend and thinks she will know the truth.
When Iago tells Othello Cassio slept with her.
If Iago had told Othello right out that they were having an affair, Othello might consider that he had a motive for saying so. It would become an issue of "Who is lying? Iago or Desdemona?" and Othello would probably believe Desdemona. But by planting the seed of suspicion and then feeding it by downplaying it, Iago makes sure that Othello is not even aware that it is Iago that is telling him about the supposed affair. He thinks he's finding it out for himself. This way it does not become a credibility issue between Desdemona and Iago.
He was a messenger, a go-between between them.
Iago tells Cassio that he must persuade Othello to reinstate him. He says "the general's wife is now our general" and so the best way to persuade Othello is to get Desdemona to plead on Cassio's behalf. Since Iago has also told Othello that Desdemona and Cassio are having an affair, the harder Desdemona pleads, the worse it is going to look for both her and Cassio.
He sees her as Desdemona's friend and thinks she will know the truth.
Iago's master plot to annoy Othello and Desdemona, is to tell Desdemona's father that Desdemona has married the Moor (Othello). And also to convince Othello that Desdemona is involved in an affair with Michael Cassio.
Brabantio is Desdemona's father. He becomes very upset when Desdemona elopes with Othello and accuses Othello of committing witchcraft in order to woo his daughter. Othello denies any wrongdoing and Brabantio warns Othello that if Desdemona deceived her father, she can deceive Othello as well. Brabantio dies from a broken-heart resulting from Desdemona's marriage.
When Iago tells Othello Cassio slept with her.
Clearly, the answer is in Act 2. Your best plan is to read/watch it. The long term consequence is probably that Cassio gets fired, although the cause-and-effect is difficult to trace.
If Iago had told Othello right out that they were having an affair, Othello might consider that he had a motive for saying so. It would become an issue of "Who is lying? Iago or Desdemona?" and Othello would probably believe Desdemona. But by planting the seed of suspicion and then feeding it by downplaying it, Iago makes sure that Othello is not even aware that it is Iago that is telling him about the supposed affair. He thinks he's finding it out for himself. This way it does not become a credibility issue between Desdemona and Iago.
He was a messenger, a go-between between them.
Iago tells Cassio that he must persuade Othello to reinstate him. He says "the general's wife is now our general" and so the best way to persuade Othello is to get Desdemona to plead on Cassio's behalf. Since Iago has also told Othello that Desdemona and Cassio are having an affair, the harder Desdemona pleads, the worse it is going to look for both her and Cassio.
If Desdemona asked a favour on Cassio's behalf, it would look to Othello like she was doing it out of an improper love for Cassio. The harder she argues, the more suspicious it seems to Othello.
Desdemona. Iago says that Desdemona is the most influential person with Othello, so Cassio should get her to plead on his behalf. This sounds reasonable, except that Iago has suggested that Cassio and Desdemona are having an affair, and this will heighten Othello's suspicions.
Othello was just out of earshot when Iago talks to Cassio. He has told Othello that he is talking to Cassio about Desdemona but he is really talking about the woman that Cassio is really having an affair with, Bianca.
He tells him that the best way for him to get his place back is to get Desdemona to plead with Othello, which would probably be good advice if Iago had not at the same time suggested to Othello that Desdemona was having an affair with Cassio. Because of Iago's suggestion, Desdemona's concern for Cassio looks like confirmation of her having an affair with him.