For Iago the dismissal of Cassio is essential to securing both his and Othello's downfall. Iago knows very well that Cassio was promoted through Othello's choice. Iago knows this painfully. Iago's plan to ruin Othello is rooted in constructing an aura of insecurity and doubt that will cause the great leader to crumble under the weight of his own second- guessing. In constructing the situation for Cassio's dismissal, Iago has been able to develop this notion of doubt within Othello.
It was Othello's decision making that promoted Cassio. In engineering the situation in which Othello is forced to see his decision to promote Cassio as questionable, Iago has succeeded in making him doubt his abilities. Iago recognizes that the only one who can destabliize Othello is Othello. In being able to generate Cassio's dismissal, Iago has been able to develop doubt in Othello's mind. This is parlayed into doubt about Desdemona as well as doubt about all of Othello's achievement. The ability to cast the destructive shadows of doubt and insecurity in Othello's mind and heart is aided with the dismissal of Cassio.