what is the purpose of this brief scene? How does this scene contribute to the "fair is foul" theme?

This scene serves as a bit of dramatic irony. The King and Banquo mention how tender the air around Macbeth's castle is yet the audience knows Duncan will be killed.

THe whole foul is fair motif is an example of equivocation, an informal logical fallacy. The King complementing the gentle air around Macbeth's castle follows ta similar line of thought.