Act I, scene i of William Shakespeare's Macbeth could be defined as one of the most intense of the entire play. This scene sets the mood of the play as a whole. The opening image of the witches chanting during an intense thunderstorm show the play to be one which will be filled with dark imagery, supernatural elements, and evil (given the stereotypical thoughts about witches).
Given that the scene sets the mood for the rest of the play, the scene is very important. Without this scene, the play would look more like one of Shakespeare's history plays (like Julius Caesar and Henry V) given the second scene of act one shows a battlefield and dialogue consists of talk of battle and valor.
Although murder and plotting extends throughout the play, it is the initial scene which puts all other action into context. It is the witches quote, "Foul is fair, and fair is foul," which sets the paradoxical nature of the play in motion. Readers and viewers are cued into the fact that nothing will be as it seems throughout the play.