Banquo is the only character who is present when Macbeth first hears the prophecy. He knows Macbeth will become king. The witches also prophesize that Banquo will beget kings. Both of these characters have something to gain in the future. This makes the scene important because it clearly shows foil characters: one good character, Banquo, who will not do anything that will not keep his conscience clear; Macbeth, the evil character who will now do whatever it takes to become king.
The two characters are used to show good and evil and how important indivdiual choice is. The witches motivate Macbeth while they do nothing to change who Banquo is. Macbeth is clearly a weak pawn dictated by the witches.