The Porter: this reminds the audience that Macbeth and Lady Macbeth, who have been the only two characters on stage for most of the last two scenes, are not the only people in the world of the play. Everyday life is still going on as usual for others.
It offers “comic relief,” a humorous break in the play that Shakespeare’s audience would have expected and enjoyed. It also gave the clown in The King’s Men, Shakespeare’s company, a role to play.