The purpose of this scene is known as "comic relief". The audience has just witnessed the killing of Duncan and the drama surrounding it and this comic scene gives the audience a chance to catch their breaths and recover from all the drama that they have just witnessed. In addition, the scene also establishes the evil atmosphere around Macbeth's castle. When the porter curses, he does not use God's name but instead uses the "name of Beelzebub", or the devil. When he hears a knock on the door, he predicts the knock is from one of three people who are all associated with hell, a farmer who committed suicide, someone who committed treason, or a thief. He even refers to people walking "to the “everlasting bonfire.” These references help to symbolize the fact that Macbeth is "raising hell" in Scotland.