What is the purpose for Act 1 Scene 1 in Macbeth? Is it to establish the mood of the play, or is it to describe the setting?

Act I scene i of Macbeth establishes
the mood, imagery, and themes of the play.  Here are some
examples:

The Supernatural:
the supernatural witches will juxtapose the natural (loyalty) and unnatural (murder)
later in the play.

Imagery:
connected to weather, women (gender), war (blood), and the number 3 (witches, "thunder,
lightning, and rain")

Time
(past v. present v. future): the play begins with a question: "When shall we three meet
again?"  Do the witches foretell the future or do they simply comment what will
inevitably
happen?

Equivocations: the
witches reveal the language of confusion; ambiguity; double meanings; half-truths;
paradoxes; riddles

“Foul is fair and fair is
foul”

Equivocal
Morality: Are the witches good, neutral, or evil?  What's the difference?
 How do we know what’s good, or who’s good, if there’s overlap between good and
evil?

Pathetic Fallacy: the
outside weather (storm) reveals and foreshadows the interior weather (mood) of the play
and the Macbeths.  They are storming with ambition and cruelty.