Why do you think that Shakespeare opens with the witches in Macbeth?
The opening of any Shakespeare play, including Macbeth, is about exposition.
In the short opening scene, Act 1.1, the following is revealed:
- The weather, thunder and lightning. This is not a "sunny" play.
- The witches: the supernatural will be prevalent.
- The witches will meet again in "thunder, lightning, or in rain?" Again, sunshine is not an option.
- A battle is going on--the hurly-burly. Definitely necessary information.
- They'll be meeting on a "heath," which has negative, wild connotations.
- They'll be meeting someone named "Macbeth."
- The witches call on spirits for help.
- Things are not as they seem or are supposed to be: what's fair is actually foul and what is foul is actually fair.
- The air is foggy and filthy.
The mood and atmosphere are set for the play here, as well as the main character and dominant themes introduced.
Concerning why witches are used instead of any other means, any answer is partly speculation. We can't go back and read Shakespeare's mind.
At the same time, the effects of using witches are evident. In addition to the list of what the witches reveal, we know that Macbeth will later be associated with the witches when he echoes the fair and foul line, and we know that the witches serve as the catalyst for the plot and the conflict. It is the predictions the witches make that ignite Macbeth on his course of action.
And, by the way, James I, the reigning monarch at the time the play was first produced, was fascinated by witches. That may have influenced Shakespeare a bit.
In addition to exposition, populist draw and atmospheric setting, one reason for starting the play "Macbeth" by William Shakespeare with the witches is the possibility that the author was trying to appeal to the natural hobbies and interests of the king - King James. It is believed by some critics that King James the monarch had had a brush with witchcraft in his childhood or earlier life and that this had left him with a driving interest in and superstition about witchcraft. France, which had a particular historical affinity with Catholic Scotland, was thought to have a strong witchcraft tradition. It was also tantamiunt to treason to ask after or even mention the well being or fate or the king.
I would say that Shakespeare opens this play with the witches in order to set a tone for the whole play. The witches are a way of symbolizing that there is going to be a great deal of evil in the play.
Just by being there, the witches show this, but they also show it by what they say. The witches talk about how fair will be foul and foul will be fair. They are saying that morality will be turned on its head and people will do evil things.
So, by their presence and by what they say, the witches set a mood or a tone for the whole play, showing that the play will be about evil.
The opening with the witches is not just a populist draw in Macbeth. The first scene is a great expository scene where without directly representing the human world, Shakespeare obliquely presents it to us through the eyes of the witches. The parallel existence of the natural and the supernatural is an interesting point made by the first scene.
The scene has great visual appeal--the witches, the vast barren heath, the thunder and rain.
The theme of equivocation, at the heart of the play is introduced through the contradictory and cryptic language of the witches.
The centrality of Macbeth's character is asserted in the promised meeting. There is great dramatic suspense built up too.
The moral maxim "Fair is foul foul is fair" can only be uttered from an outside to the natural and the human as a contamination which would pollute Macbeth's speech before the meeting in the act I scene III.