In this play one of the main purposes of Banquo's character is to act as a contrast to the character of Macbeth. Banquo is brave and noble - characteristics that Macbeth arguably doesn't possess. Interestingly, like Macbeth, Banquo is ambitious, but signficantly, unlike Macbeth, Banquo does not act on those ambitious thoughts to convert them into action. If you think about this comparison a bit further it is clear that Banquo's character stands in the play for a path that Macbeth did not take, and acts as a reminder that ambitiou by itself does not necessarily have to be translated into treachery and assassination. We can see therefore why it is Banquo's ghost (and not the ghost of Duncan) that haunts Macbeth and why this haunting is so powerful. The ghost interestingly reminds Macbeth that Banquo did not copy Macbeth's response to the witches' prophecy.