What is the purpose and effect of Duncan's and Banquo's comment when they approach Macbeth's castle?

This scene serves two purposes; first, it gives insight into the characters of Duncan and Banquo, and how they view Macbeth. Secondly, it is ironic (and symbolic) in nature as Macbeth is not with them, but had ridden ahead to reach the castle before them. They tell Lady Macbeth that he'd ridden ahead to be with her. She welcomes them into the castle and acts the perfect hostess. This is, of course, more foul than fair because she has no one's goodwill in mind, nor does Macbeth.