What is the significance of the first scene in Macbeth?

The first scene of Macbeth
establishes the following:

• establishes the theme
  of "time": "when shall we three meet
  again?"


• establishes the importance of the "heath": a
  place of carnage, full of "foul and filthy air," foreshadowing the bloody events to
  come


• establishes the witches as agents of chaos,
  seemingly controlling the actions and events of play much like a
  chorus


• creates an atmosphere of the supernatural,
  mystery, and the occult


• pathetic fallacy: the outside
  weather (stormy) mirrors the interior weather of the Macbeths
  (murderous)


• the motif of "three": three witches,
  "thunder, lightning, rain"


• the language of confusion in
  the witches' equivocations (language of confusion; ambiguity; double meanings;
  half-truths; paradoxes; riddles: “Foul is fair and fair is
  foul”)


• subverts the natural order (God, King, and nature
  as all good) with equivocal morality: how do we know what’s good, or who’s good, if
  there’s overlap between good and evil?