Final answer:
Iago tells Roderigo to start a fight with Cassio as part of a scheme to tarnish Cassio's reputation and remove him from a position of favor with Othello, thus advancing Iago's own interests.
Step-by-step explanation:
Iago tells Roderigo to start a fight with Cassio because Iago wants to use this situation to get rid of Cassio. By instigating a fight, Iago's plan is to tarnish Cassio's reputation and cause him to fall from Othello's favor, thereby creating an opportunity for Iago to promote his own self-interests. Iago is a master manipulator and uses Roderigo's willingness to take action as a way to advance his scheme against Cassio, taking advantage of Roderigo's feelings and loyalty. This is a key example of the theme of manipulation in Shakespeare's Othello.