In William Shalespeare's play Macbeth, the three witches are very important. They set the mood, set the plot, and to bring the supernatural into the play.
First, the witches set the mood of the entire play. Given the witches are seen in the opening scene of the first act, the mood is defined as ominous and dark.
Second, the witches are important for their actual role in the play. The witches are responsible for providing Macbeth with the prophecies. Without these prophecies, Macbeth's ambition would not have ruled him (which would also have kept him from murdering Duncan and being killed himself).
Lastly, the witches add a supernatural element to the play. Shakespeare wrote Macbeth for King James I. King James was highly superstitious and enjoyed the supernatural. Therefore, the inclusion of the witches in the play offered homage to the king.
In Macbeth, the witches were very important in making his decision to take the throne of Scotland from Duncan. Macbeth had this idea prior to meeting up with the witches but once they told their prophecy it was confirmed in his mind. Once Macbeth tells his wife about the prophecy, she becomes a driving force in his striving for power. Without the meeting with the witches, Macbeth probably would not have had the strength to murder Duncan. The entire shape of the story would be different if the witches were excluded.
They are the important because they intiated the events that will determine Macbeths future. (tragdey)
Without the witches, Macbeth will be simply seen as a brave and couragous Thane of Cawdor, which we all thought he was until he met the witches. This shows Macbeth's other side, which is doubtful and just not as brave as we thought in general.
The witches are the catalyst that ignite the fire of ambition in Macbeth and ultimately spark the action that occurs. Therefore making them a vital element to the action.
On a heath, with thunder still rumbling, the three witches are awaiting Macbeth on his way from victory. when Macbeth, accompanied by Banquo arrives,the witches greet him as "thane of Glamis", "thane of Cawdor", and "king hereafter". They disappear before the arrival of Ross and Angus who bring the news that the king has bestowed on Macbeth the title of "thane of cawdor". Fascinated by this speedy proof of the witches' foreknowledge, Macbeth is 'rapt' and he begins to speculate to himself upon the prospect of becoming the king in future.He accepts the prophecy as an invitation to hellish deed of evil.
Here the world of supernatural and the world of men, which remained separated till now, are brought together. The vindictiveness of the witches apparent in their curses on the sea captain, casts a gloom on Macbeth's carrer also.
The prophecies of the witches are clearly an influence, dangerous circumstance with which Macbeth has to deal.But Macbeth was not an innocent man-therefore he is startled and agitated. He was free to accept or reject the soliciting but the temptation was already within him. He speaks of "supernatural soliciting" but in reality they just announce events, they do not tempt. The idea of fulfilling the prophecy by murder was entirely of his own.
For example- In Act 4 sc1 Macbeth seeks out the witches, as he had commited himself to his course of evil. it is now that they do 'solicit'. They prophesy, but they also give advice: they bid him to be 'bloody', 'bold' and 'secure'. Even then he is free to act on his own initiative. His first act after the meeting- the slaughter of Macduff's family was not hinted by them.
So the weird sisters may be anything except fates. They have no direct share in the action and yet form an integral part of the play. They and their prophecies represent not only the evil tendencies in the hero's soul but also the unknown vague influences of evil around him in the world which help the incitements of his wife and his own ambitions.