In The Age of Shakespeare, editor Boris Ford elegantly explains why this aside is a defining moment in the play:
...the words do not only point inward to the presumed state of Macbeth's mind but, as it were, outward to the play as a whole. The equivocal nature of temptation, the commerce with phantoms consequent upon false choice, the resulting sense of unreality ('nothing is but what is not'), which has yet such power to 'smother' vital function, the unnaturalness of evil ('against the use of nature'), and the relation between disintegration in the individual ('my single state of man') and disorder in the larger social organism - all these are major themes of the play which are mirrored in the speech under consideration." (Ford, Boris., ed. The Age of Shakespeare. Middlesex. Penguin Books Ltd., 1955, [p.p. 230])
Mabillard, Amanda. Macbeth Glossary. Shakespeare Online. 20 Aug. 2000. (date when you accessed the information) < http://www.shakespeare-online.com/plays /macbeth/macbethglossary/macbeth1_1/macbethglos_twotruths.html >.