Act I of Othello is essential to the audience's understanding of the play, its characters and its themes.
Act I Scene i introduces the antagonist, Iago. We learn of his hatred of Othello, his manipulation of Roderigo and that he is duplicitous:
"I am not what I am."
The plot device of the partial conversation, which opens the play, hooks the audience, as does the references to Othello without mentioning him by name. In Iago's language, we are shown the typical Jacobean attitude towards Black or non-Christian people, Shakespeare plays on the prejudices which the audience may hold. This creates curiosity.
We see Othello for the first time in Act I Scene ii, with Iago. Iago's behaviour here demonstrates that he is not to be trusted, while Othello appears to contradict the initial impression the audience has been given.
Act I Scene iii allows us to see how Othello is viewed by the nobility of Venice as well as giving him and Desdemona to explain their love for each other.
In this opening act we are introduced to the majority of the main characters as well as the central point of conflict within the play.
Also, as a second thought, he gains the audience's support in the first scene by making them laugh. I think it's important that we view Iago with a certain admiration. He is clearly engaging and charismatic and our feeling of being implicit in his actions gives the climax of the play more dramatic weight and, arguably, heightens the feeling of catharsis since we feel somewhat guilty.
The first act was important in showing Iago's intentions. Not only was he a comic device to make the audience laugh when talking about "the black ram is tupping your white ewe" but also to emphasize his role as the dramatic villain. He is presented with a sence of charisma which becomes paramount to his character and it is established that he has a twisted obsession with the image of Othello and Desdemona having sex ("making the beast with two backs..") which is a recurring theme throughout the play. It also, I think, shows his manipulative talents which he clearly uses in the first scene.