The major theme of Macbeth is the battle between good and evil and the other themes are connected to this so it seems only fitting to begin the play with the witches.
Nothing is ever as it seems in Macbeth and the witches famous words
Fair is foul, and foul is fair
already draw the audience into the concept of appearance versus reality and that there may be more than meets the eye.
Much of the action throughout Macbeth relates to Macbeth’s obsession with the witches and bringing them in at the onset leaves us (the audience) in no doubt as to the influence they are likely to have over the main character.
By the time the witches meet Macbeth, the audience is already aware that Duncan intends to make Macbeth Thane of Cawdor. It was quite acceptable and actually expected in the day that, having served his king loyally and bravely, Macbeth would be richly rewarded.
The audience is again drawn into the witches scheming when they appear shortly after Duncan's announcement and, of course, before Macbeth actually knows himself, to prophesy that Macbeth will be honored by the title of Thane of Cawdor.
Macbeth’s first words
"So foul and fair a day I have not seen," alludes to the initial prophecy of the Three Witches.
Even though, Macbeth and Banquo question the evil nature of the witches, Macbeth cannot help but be intrigued by the fact that, not only will he be Thane of Cawdor but King! He begins to believe it and the audience realises that maybe his rise to power will not be according to the proper order but by evil and devious means.
The eNotes study guide will help you get a better understanding of the role of all the characters. Navigate to the summaries and character analyses.