Act III, Scene IV, is important because it is Macbeth's high point as King. Once he sees the ghost, his image as king is changed, tarnished with questions of madness.
"Then comes my fit again: I had else
Whole as the marble, founded as the rock,
As broad and general as the casing air:
But now I am cabin'd, cribb'd, confin'd, bound in
To saucy doubts and fears. But Banquo's safe?" (Act III, Scene IV)
"Can such things be
And overcome us likes a summer's cloud,
Without our special wonder? You make me
Even to the disposition that I owe,
When now I think you can behold such sights,
And keep the natural ruby of your cheeks,
When mine are blanch'd with fear." (Act III, Scene IV)
Macbeth begins to question his sanity, he can't believe his eyes, yet he cannot look away from Banquo's ghost. In front of his dinner guests, he acts in an unstable, irrational manner, causing Lady Macbeth to make excuses for his behavior. At this point, King Macbeth has lost some of the respect and admiration of his court.
His subjects do not look at him the same way after this scene, it is a turning point for Macbeth. His manner and attitude becomes more tyrannical, he decides after this scene to consult the witches again, to seek their guidance.
Macbeth begins the slow descent into madness after this scene, losing his ability to control the future, something that he has killed to achieve.
Act III, Sc. iv (commonly known as the Banquet Scene) is important in two directions. Firstly it gives a clear picture of Macbeth’s psyche. Secondly it helps to create the supernatural atmosphere of Macbeth. When Macbeth speaks to the ghost of Banquo (who however cannot reply) the audience knows how deeply Macbeth has been suffering from the feeling of guilt. The scene also projects how Lady Macbeth continuously tries to help Macbeth in difficult situation. The eNotes commentary of this scene is an extremely useful resource and should be studied for a thorough understanding.