What is the Purpose and effect of Duncan's and banquo's comments when they approach Macbeth' s castle

The dramatically ironic comments by Duncan and Banquo when they approach Macbeth's castle in Shakespeare's Macbeth serve numerous purposes. One of the most important is the reinforcement of a central theme first introduced by the witches and later echoed by Macbeth: the difference between appearance and reality--what's fair is foul and what's foul is fair.

Duncan and Banquo interpret the appearance of the castle and its surroundings as if a wholesome, noble, and faithful family ...