Macbeth was written for James I, who was known to be descended from Banquo, and therefore portrays him as the rightful heir to the Scottish throne.
I think that you can take this idea in many different directions. On the one hand, Macbeth is known to have been either commissioned for King James I or written to honor him. Either way, it is clear that the play makes Banquo look good, and Macbeth look bad. First of all, the witches make a clear prophecy ab out Banquo’s line that shows that his sons will become king. This is what scares Macbeth enough to have him killed, even though he is a loyal friend.
Lesser than Macbeth, and greater.
Not so happy, yet much happier.
Thou shalt get kings, though thou be none.(70)
So all hail, Macbeth and Banquo! (Act 1, Scene 3)
It is interesting that the first witch describes Banquo as both lesser than Macbeth and greater. She is saying that since he is not going to become king himself, he is lesser than Macbeth. However, since his sons are the ones who will ultimately become the heirs, he is greater.
Banquo gets a leg up on Macbeth during the entire play. He even comes back from the grave, as a ghost! You could say he gets the last laugh. His ghost appears at Macbeth’s banquet and makes a fool of Macbeth.
The table's full.(55)
Here is a place reserved, sir.
Thou canst not say I did it: never shake
Thy gory locks at me.
ROSS: Gentlemen, rise; his Highness is not well. (Act 3, Scene 4)
So even though Banquo is dead, he is still noble, still dignified, and still more respectable than Macbeth, who is slowly losing it. He reappears again in the scene with the mirrors, which further panics Macbeth. This is where Macbeth goes again to consult the witches in Act 4, Scene 1. The mirrors show that Banquo’s line will continue on and on, and the witches’ prophecy will reign true. Macbeth is doomed, and he realizes it. This demonstrates Shakespeare’s political prowess, because it shows again the lasting Banquo line, which is James I’s line. So while Macbeth is entertainment to us today, it was a bit of politics then.