This passage from Act I, Scene 3 acts as foreshadowing and, also, as an insight into how the character of Banquo responds to the preternatural element of the Elizabethan drama.
In this scene Banquo and Macbeth cross the heath from the battlefield as they are on their way to the court of King Duncan at Forres. The third witch hears them approaching and calls out to the other two, thus introducing the element of the spiritual. When he sees them, Banquo asks if they are mortal beings, observing that they do not appear to be “inhabitants o’ th’ earth” (1.3.39).
Further, as he addresses the witches, Banquo displays his skepticism of their fortune-telling. Having heard what they have predicted for Macbeth, Banquo also observes how stunned, yet intrigued, Macbeth is. But he challenges the witches, saying that if they truly can "...look into the seeds of time / And say which grain will grow and which will not," then they should tell him what his future holds too (1.3.60-61).
Thus, the above passage is significant because it provides insight into Macbeth and Banquo's characters: Macbeth is clearly moved and influenced by the three "weird sisters" from the preternatural world while Banquo is not.