Fundamentally, what Cassio serves to enlighten is the corruptive power of reputation within Jacobean society. By the end of the play, Shakespeare has shown the audience that Cassio is not who they were originally led to believe he was. He mistreats Bianca, takes advantage of Desdemona's kindness and thus plays a part in her downfall, and becomes a coward when stripped of his position of lieutenant. However, upon the plays close, Cassio is appointed leader of Cyprus and is responsible for Iago's punishment.
This could perhaps show how the status quo is always restored, as society refuses to deviate from strict conventions and it's delineated ideas of reputation, despite the tragedy it fuels. It is this determination to resist change which Shakespeare ventures to critique, spurring his audience to re-evaluate the conventions they hold most dear