The witches in Shakespeare's Macbeth effectively introduce the play in numerous ways. They add suspense. Answering the question when they will meet again the second witch answers:
When the hurly-burly's done,
When the battle's lost and won.
Thus, a battle is introduced and the audience has to wonder what battle, who's in it, and who's going to win?
They add mystery. Together with the weather--thunder and lightning--their plan to meet next on the "heath" is suggestive. A heath in literature is a forbidding place, a barren place, a place of mystery. This, of course, also adds to the suspense.
The witches also introduce the main character/tragic figure, Macbeth: "There to meet with Macbeth."
Finally, Shakespeare begins the play with the plot already "in motion," so to speak, by showing the audience the conclusion of a meeting that is in the process of occurring when the play opens.
Plus, the witches are just plain entertaining, and, by the way, King James I, the monarch and art patron the play would have been performed for, would have probably loved it. He was interested/fascinated with witchcraft.
As Shakespeare scholar David Bevington notes, Macbeth is a play "concerned with spiritual evil, as distinguished from domestic or political strife," a play that "offers a terse and gloomy vision of a particular man's encounter with the power of darkness" (The Complete Works of Shakespeare, p. 1043).
It is therefore fitting that such a thematically dark play, an exploration of evil, opens with witches, supernatural creatures of evil, whose presence sets the tone for the tale that will unfold. We meet them amid thunder and lightning, in a stormy, foreboding landscape. While they speak of the "hurly-burly" or tumult of a particular battle being won or lost, the battle that earns Macbeth the title of Thane of Cawdor, their words also foreshadow the larger battle with evil that will be won or lost in Macbeth's soul.
The witches in the opening scene say they are on the heath "to meet with Macbeth." As an audience, we are being warned from the very start of the action that Macbeth has been targeted for an encounter with evil. The witches end the short scene introducing another major theme of the play: that "fair is foul, and foul is fair." In other words, nothing is at is seems. They also introduce the theme of corruption, as they state that they (creatures of darkness) will hover in the "filthy air."
Opening with the witches, Shakespeare clearly cues his audience that this will not be a happy, light-hearted pastoral comedy but instead an encounter with the dark side, a play about deception and corruption.