As a moor, Othello would have already felt somewhat out of place. He might have already harbored some of the doubts that Iago plays into. He might have already questioned the loyalty of a woman to him. It also assists in the reaction of Desdemona's father. Iago is able to stir up trouble quickly because there is some prejudice against Othello already. Othello didn't have to be a Moor, but he did need to be apart from the others in some way. He needed to have experienced prejudice and have the seeds of doubt in place. Setting Othello as Moor was a clever and easy way to shape the story with a more believable plot.