evil. Or, it could also be argued that the witches are figments of Macbeth's already rotten inner desires. In any case, the general story is just as the witches predicted in the opening scene; Macbeth, who once was fair, grows more and more foul throughout the play.

In Act I, when Macbeth first hears the three witches welcoming him on his arrival: Thane of Glamis, Thane of Cawdor, and king hereafter, he doesn’t immediately believe that there is any truth to their words. But when he’s immediately greeted and given word of his new title by Ross, suddenly the thought of becoming king is sounding more likely every minute. Of course, initially, Macbeth is not viewed as a violent person, a person who could actually make it happen; he’s more likely to bide his time until Duncan dies and gives Macbeth the throne in a more righteous manner (even when this isn’t guaranteed).

On the other hand, Lady Macbeth’s first thoughts when she hears the news are the exact opposite of her husband’s. She thinks that although it would be great for Macbeth himself if he were to be king, it would be even better for herself if she were to be the wife of the king, and receive all the perks that come with the job. She’s very influential in getting Macbeth to want those same things, and the greed eventually causes his unravelling.

And with that, Macbeth kills the king and becomes king himself.

Therefore, when Macbeth sees the three apparitions, he believes that they are true, because so far, his experiences with the witches have given him no reason not to. Macbeth’s greed overpowers his ability to be rational, and if he believes the apparitions are true, he feels secure knowing he can’t be defeated. When it becomes clear that the apparitions are not coming true, his pride gets in the way of acknowledging the truth, and finally, when even his pride gives in to the truth, he continues to hold on to that belief because in the end, everyone needs to have some hope to hold on to, and those 3 apparitions are all that he has left.

In Macbeth by William Shakespeare, the eponymous hero Macbeth is initially skeptical concerning the validity of the three witches' predictions. Despite this, he is intrigued, as they predict his becoming a king. Part of the initial mechanism is roughly the same one that even today causes people to believe in astrology or psychics when they predict good things, that all of us would like some assurance of good things happening in our future.

The point at which Macbeth starts to trust the witches rather than just having his interest piqued is when he first sees their predictions come true. After his meeting with the three sisters, he does become Thane of Cawdor. When this happens, confirming the truth of their prophetic abilities, he also believes their other prediction that he will become king. This prediction is, in a sense, self-fulfilling, as it stirs within Macbeth the ambition that makes him kill Duncan to become king. This act, of course, gives him additional confirmation of their prophetic abilities, leading him to believe their final set of prophecies, which also come to pass. 

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.

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.

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.

Well, first he does, then he doesn't, then he does. Sort of like a fish nibbling at the bait...

When Macbeth first meets the witches, he is overcome by the possibilities presented to him (Act 1, scene 3):

MACBETH:

[Aside.] Two truths are told,

As happy prologues to the swelling act

Of the imperial theme!—I thank you, gentlemen.

[Aside.] This supernatural soliciting

Cannot be ill, cannot be good. If ill,

Why hath it given me earnest of success,

Commencing in a truth? I am Thane of Cawdor.

If good, why do I yield to that suggestion

Whose horrid image doth unfix my hair

And make my seated heart knock at my ribs,

Against the use of nature? Present fears

Are less than horrible imaginings:

My thought, whose murder yet is but fantastical,

Shakes so my single state of man that function

Is smother'd in surmise, and nothing is

But what is not.

Yes, he is moved to believe in them and the future that they offer him and is already thinking of murder. Then, quickly, he has second thoughts and decides maybe things will happen the way they happen; he'll just wait and see:

MACBETH:

[Aside.] If chance will have me king, why, chance may crown me

Without my stir.

Along those same lines, later in scene 7, Macbeth figures he has it good enough, he's still the captain of his fate, and the heck with what the witches say; he'll just keep things as they are:

MACBETH:

If it were done when ’tis done, then ’twere well

It were done quickly. If the assassination

Could trammel up the consequence, and catch,

With his surcease, success; that but this blow

Might be the be-all and the end-all here,

But here, upon this bank and shoal of time,

We'd jump the life to come. But in these cases

We still have judgement here, that we but teach

Bloody instructions, which being taught return

To plague the inventor. This even-handed justice

Commends the ingredients of our poison'd chalice

To our own lips. He's here in double trust:

First, as I am his kinsman and his subject,

Strong both against the deed; then, as his host,

Who should against his murderer shut the door,

Not bear the knife myself. Besides, this Duncan

Hath borne his faculties so meek, hath been

So clear in his great office, that his virtues

Will plead like angels trumpet-tongued against

The deep damnation of his taking-off,

And pity, like a naked new-born babe,

Striding the blast, or heaven's cherubin horsed

Upon the sightless couriers of the air,

Shall blow the horrid deed in every eye,

That tears shall drown the wind. I have no spur

To prick the sides of my intent, but only

Vaulting ambition, which o'erleaps

And falls on the other—

Then in comes Lady Macbeth and she chastises him with her tongue and convinces him to believe again in the witches' prophecy and promise of greatness. Finally, he agrees:

MACBETH:

Bring forth men-children only,

For thy undaunted mettle should compose

Nothing but males. Will it not be received,

When we have mark'd with blood those sleepy two

Of his own chamber, and used their very daggers,

That they have done't?

So, he's back again and says, at the end of the act:

I am settled, and bend up

Each corporal agent to this terrible feat.

Away, and mock the time with fairest show:

False face must hide what the false heart doth know.

Now he's caught.

People did believe in witches, but as you can tell they were also afraid of them.  Witchcraft is fun for us in modern times, and clearly there was a certain amount of fun in it in Shakespeare’s day, but there was also more fear.

I think you will find this review that I wrote a couple of months ago very enlightening:  Witches and Jesuits:  Shakespeare's Macbeth.   http://www.enotes.com/blogs/book-blog/2008-07/witches-and-jesuits/

If you are able, pick up a copy of the text.  It will help you understand how integral witches are to the play. 

Keep in mind, that the belief in witchcraft is still popular today.  It is not a mainstream belief, but there are individuals who practice whats known as "wicca" and there are stores and websites devoted to the study of this "craft".  The belief that people can control an uncontrollable world can be very tempting to some.  And the ability to explain away the unfortunate - the death of a young child, the misbehavior of a teenager - can also be tempting.  The strong religious beliefs at the time made witchcraft - Devil association - the likely outlet for such needs. 

However, the belief in witchcraft was not as widespread, particularly in England, as some would suggest.  Even the characters in the book has some hesistancy regarding the "witches" they meet.  Superstitious Macbeth is so eager for power that he responds instantly to the "prophecy", but Banquo is amused and disbelieving:

Banquo is asking, teasingly, if he and Macbeth have taken some drug that has caused them to halluncinate.  He doesn't buy what he has seen and heard.

This was true in much of England.  Witch persecution was much less prevalent in England than in the rest of Europe, at least until King James I took over.  James was infamously superstitious and scared of witches.

First of all, this was during a pre-scientific era in history.  While science was making some inroads on superstition, scientific causes of events was not a focus.  Psychosomatic results from a "curse" were proof of the power of witchcraft.

Also, through the teachings of the Church, the powers of evil (through Satan) were considered the cause of much that was wrong in society.  The fear and superstition caused by lack of education to fully understand this teaching, caused people to believe that individuals could call upon the devil to work in their favor. 

There was also some influence on this belief in the remains of pagan/nature religions.  These were often made a part of Christian belief, and seen as evil. 

Macbeth believes that the witches are supernatural beings because he assumes that "they have more in them than mortal knowledge." He regards them as the agents of fate as soon as their first prediction turns out to be true. When the title of Cawdor is transferred to Macbeth, he is convinced that the rest of their prophecy will come true -- that he is to become the king:

  Two truths are told,  As happy prologues to the swelling act  Of the imperial theme.

Macbeth is blinded by his own ambition to become the king, so he takes what the witches say very seriously and decides to commit himself to make their prophecy possible. He constantly depends on the witches and has succumbed to the power of their prophecy:

 I burned in desire to question them further...

He fails to see them as the masters of manipulation and naively believes in anything they say. However, they should not be the ones to blame for his downfall because Macbeth himself committed all the horrible deeds. The witches did not make him do anything.

Without the Witches, Shakespeare's Macbeth would have been a very different story.  The Witches are important for many reasons, but let's take a look at two very good reasons why they were involved.

1. The Witches provide one of three essential elements to the exposition of the play. In England when Shakespeare was writing plays, his audience would have expected that his plays begin with one of three elements: violence, sexual innuendo, and/or supernatural occurrences. This last one--the three Witches--begin Macbeth, and Shakespeare's audience, regardless of class status, would have completely believed in the possibility of Witches (and consequently, their influence on people and situations).  The Three Weird Sisters "supernatural soliciting" would have drawn the audience in, but it also would have been very interesting to one audience member in particular: King James I (the King of England, who undoubtedly was present at the Globe Theatre when Macbeth ran).  He had written and published a very famous book about witchcraft called Daemonologie. Therefore, the Witches were socially relevant to the everyday lives of Shakespeare's audience AND a personal interest of the king himself.  Shakespeare was no fool--if the king was interested in and liked his play, shouldn't everyone?
2. Now on to the plot...Why are the Witches involved in the action of the story?  They provide a very interesting, debatable element for us, the audience: blame. Whose fault is it that Macbeth transforms himself from a war hero to a homicidal, self-involved tyrant by the end of the play? Lady Macbeth, you say?  She did give him quite a push at the right time, didn't she?  What about Macbeth, himself?  Well, that is another argument, and you'd have a valid point if you made it.  However, what if the responsibility for the entirety of Macbeth's problems stemmed from the Witches.  That complicates things a bit, doesn't it?  The Witches prompt us to consider whether Macbeth created his own fate or was doomed the very moment he stepped upon the heath with Banquo and received his first prophecy.  Remember, the Witches say in Act I, sc. i that the war will end soon and that they will meet Macbeth upon the heath.  Both of these comments show that they can foretell the future. The later prophecies (the Apparitions) prove this more fully. However, one could argue (as Banquo does prior to his death) that often 

   to win us to our harm,/The instruments of darkness tell us truths, Win us with honest trifles, to betray's/In deepest consequence.

   If Banquo is right, and the Witches are merely toying with Macbeth in order to make him do terrible things, then isn't he much more responsible for his own downfall than they?  For this reason, the Witches presence provides the audience (whether during Shakespeare's time or now) something to consider with regard to fault.