Why does Shakespeare include this scene of the king interacting with Macbeth and Banquo?

This scene is important because it is meant to show the King's devotion towards Macbeth and Banquo now that the war is won. Macbeth's newly found fame and favour of the King is juxtaposed with Macbeth's internal conflict over his newly found obsession to be king. Duncan announces his decision to make his son Malcolm the heir to the throne of Scotland (something that would not have happened automatically, since his position was elected and not inherited).In an aside, Macbeth vows not to let this happen. This scene highlights Macbeth's increasing internal contradictions and torment over what the Weird Sisters had told him.