What is the significance of Macbeth's absence in act 1, scene 6?
In this scene, Duncan and his men arrive at Macbeth's castle and are greeted by Lady Macbeth. Macbeth's absence heightens the dramatic irony of Duncan's entrance. In dramatic irony, the reader or viewer has information that the characters do not. We know, from the prior scene, that Macbeth has ridden home ahead of the king to consult with his wife about murdering Duncan.
Thus, King Duncan's comments about the pleasant aspect and "sweet" air of Macbeth's castle being pleasing to his "gentle" sense drips with irony because we know that Macbeth and Lady Macbeth are plotting his murder. Banquo echoes Duncan's sentiments, saying the "air is delicate" and praising the birds.
Having Macbeth absent highlights Lady Macbeth's treachery, allowing her to step to the forefront and reveal her deceptive ways as she deceives her guests with false hospitality, pretending that the castle will offer them every honor and protection.
There's also irony in Duncan's assumption that Macbeth took off ahead of them, riding so quickly they couldn't keep up, out of his "love" for Duncan, interpreting this haste as Macbeth's desire to prepare for the visit. He doesn't understand that Macbeth's "preparation" for his stay is to plot with his wife to murder him. Macbeth's absence allows Duncan all the more freedom to interpret events as he wants them to be.