The witches in William Shakespeare's Macbeth are key characters that serve as the impetus for Macbeth's ambition and his eventual decision to commit murder. The witches, who appear at the very opening scene, give Macbeth five prophecies that spark his ambition and set into motion the key events of the tragedy.
These witches are present at the very outset of the play where they are waiting for Macbeth to meet. The weather is very foul and rough. But all of them say,
"Fair is foul, and foul is fair"
The above line sets the nature and mood of these weird sisters. It also expresses their unhuman trait. Unlike any human they feel it much favorable when it is foul for others and uncomfortable when it is comfortable for others.
The first time Macbeth and his companion, Banquo, encounter the witches, we see a glimpse of the witches' overall look and impression through the words of Banquo. He says,
"How far is't call'd to Forres? What are these
So wither'd and so wild in their attire,
That look not like the inhabitants o' the earth,"
The witches are the supernatural being in the play; they have the appearance like women, and yet they don't look like inhabitants of the earth. Banquo also realizes that it is difficult to identify their gender because of their beards.
The witches are notable in this play mostly because of the prophecies they made. These prophecies awakened the evil in Macbeth. Though the ambition to become the king is already harbored in Macbeth's mind prior to this but it is strongly uncertain whether he would proceed to his ambition if the witches do not make prophecy regarding this. To Macbeth, it is like the divine signal and approval to his concealed plan.
First Witch says "All hail, Macbeth, hail to thee, Thane of Glamis!"
Second Witch says," All hail, Macbeth, hail to thee, Thane of Cawdor!"
Third Witch says, "All hail, Macbeth, that shalt be King hereafter!..."
At the time, Macbeth is Thane of Glamis (a Thane is a titled landholder), but the witches prophesy that he will not only be the Thane of Cawdor but also king of Scotland which stuns his senses.
Upon hearing this, Macbeth is not convinced that it will come true, but his wife, Lady Macbeth, has no doubt of its truth. It is her ambition that first ignites him to take drastic action in order to secure the titles the witches allude to.
In fine, the witches play a vital role, as the catalyst of the final tragedy of Macbeth. Without the notorious presence of these witches, the evil plan of Macbeth might have been kept buried inside. They are quite instrumental in Shakespeare's tragic play 'Macbeth'.