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 live there. The air is sweet, etc. But just as the traitor Macdonwald was a man Duncan put absolute trust in, so is Macbeth, and both the king and Banquo misjudge him.
In fact, the castle is compared to hell by the porter and Lady Macbeth invokes the help of spirits to make her more evil and mortally aggressive. While the Macbeths appear to be acting as hosts, they are really planning an assassination.
The Macbeths are not wholesome and noble, and are faithful only to evil "spirits" and their own ambition.
The misguided comments by Duncan and Banquo demonstrate that they are clueless as to what the Macbeths are really like: they don't realize that though they appear to be fair, they are really foul.