George Kittredge, the great Shakespeare scholar from Boston, in writing on Macbeth, gave all the credit for the existence of the Porter to the requirements that Shakespeare needed to fulfill at that particular point in the play: He needed a character to fill a period of time between the murder and the re-entry of the cleaned-up Macbeths, but it couldn't be a major character and it couldn't advance the story. The play also needed relief from the "extreme tensions" of the bloody regicide and the discovery of the body by Macduff et al, and the type of relief had to be of a dark comic variety. Thus the Porter was born. It's an intriguing theory - as it shows how Shakespeare's understanding of what the audience would need at a given point may have been crucial to the creation of some of his great minor characters. Others of this type that come immediately to mind: the Apothecary in Romeo and Juliet, the Gravediggers in Hamlet, the Clown (carrying the asp and figs) in Antony and Cleopatra, and the Gardener in Richard II. If the theory is true, and necessity was integral to their development, it shows once more Shakespeare's unparalleled emphasis on detail: even the smallest characters in the plays are fascinating and believable.