What dramatic purpose is served by the appearance of Banquo's ghost?
I understand that Banquo's ghost rattles Macbeth deeply, causing him to look mentally disturbed to the guests at the banquet as well as makes him feel guilty for the deed he commited. I don't fully understand the dramatic purpose.
Shakespeare liked using ghosts. He uses Hamlet's father's ghost several times, and he has Caesar's ghost visit Brutus in his tent before the Battle of Philippi. Banquo's ghost serves to remind Macbeth of one of his principal concerns, which is that it is Banquo's heirs who will benefit from the murder of Duncan. Right after the banquet breaks up, with all the attendant lords having learned of Macbeth's guilt from his own lips, Macbeth decides to revisit the Weird Sisters.
More shall they speak, for now I am bent to know
By the worst means the worst.
Shakespeare wants to employ the three witches again because they create a strong emotional effect on his audience and because they will give Macbeth encouragement to become even more ruthless and tyrannical. His next encounter with the witches is the most spectacular. He receives assurance from apparitions that no man of woman born shall harm him and that he will be secure until Birnam Wood moves to Dunsinane. But in answer to his unspoken question about Banquo, he is shown eight kings of Scotland who are all descendants of Banquo.
The dramatic purpose is that it was the appearance of Banquo's ghost at the banquet that motivated Macbeth to go back to the Weird Sisters and enabled Shakespeare to put on the spectacular display of apparitions in Act 4, Scene 1.