The dramatic significance of this opening scene cannot be underestimated. It serves to set the tone and the mood for the rest of the play as the dark, ominous and foreboding presence of the witches inject a strong element of the supernatural into the subsequent action and also the way that they represent a perversion of truth and riddles. Note the way that this is suggested from the opening scene:

Fair is foul, and foul is fair,

Hover through the fog and filthy air.

Note only does the alliteration of the "f" sound successfully manage to convey significant menace and terror, but note how the witches' influence manages to transform what is "foul" into an appearance of "fairness" and vice versa. This is of course a key theme for the play, as those who appear to be good and virtuous are often the opposite, as is shown in Lady Macbeth most clearly. The opening scene of this play therefore places the witches as central actors. It is important to note that the audience does not even meet Macbeth until Act I scene 3. He is connected however with these agents of darkness through their prediction of the meeting that they will have with him, linking his character to their machinations and evil miasma that hangs over the entire play.