In Act II, Iago lies to Roderigo about Desdemona and Cassio. He convinces Roderigo that Desdemona is cheating on Othello with Cassio. This leads to Iago convincing Roderigo to get into a fight with Cassio in the hopes of getting Cassio into trouble, and it works.
"Cassio knows you not. I'll not be far from you. Do you find some occasion to anger Cassio, either by speaking too loud, or tainting his discipline, or from what other course you please, which the time shall more favorably minister" (Act II).
Basically, Iago simply uses the unassuming Roderigo as a pawn in this larger game of chess that he is playing. He knows that Roderigo will be easily swayed and do Iago's bidding. He needs to make Othello dislike Cassio and get Cassio indebted to Desdemona in the hopes of getting his job back.
Ultimately, Roderigo does get into a fight with Cassio, Iago tells Othello, and Cassio is fired. Poor Cassio's misfortunate does not simply end there, though, as Iago continues to advance the rumor that he is cheating with Desdemona, even telling Othello this straight to his face.