Iago at the outset of the play is Othello's long-serving ensign, he has been with the general in many battles and has served him faithfully to the point where Othello trusts him completely.
As the action of the play winds along, Iago becomes less the simply peeved suboordinate and more a wildly scheming monster looking to destroy Othello in the most vicious way possible. His soliloquies serve to illustrate his dastardly nature and the way that he justifies his pursuit of Othello's destruction.
Throughout the play he is adept at manipulating other characters, particularly Roderigo, to achieve his desired ends. He is typically loud and wordy but by the end of the action of the play he has been silenced as his objective has been achieved.