The witches serve to move the plot along in Macbeth.
They are the cause of all of the chaos. Even if Macbeth wanted to act on his desire to be king, he was barely aware of it until the witches gave him their first set of prophecies. They state that they are interested in messing with him, and even though we find out later that they did not really have permission from Hecate, the witches egg Macbeth on.
When Macbeth goes before Duncan, he expects to be named king—but only because the witches have planted the seed in his head.
[Aside.] The Prince of Cumberland! That is a step(55)
On which I must fall down, or else o'erleap,
For in my way it lies. Act 1, Scene 4)
Suddenly, Macbeth is not at all concerned about being dressed in “borrow'd robes”. He is fully read to be king, even though he has no right to it.
Things seem to be in better shape once Macbeth is king, but the prophecy that Banquo’s sons will be kings keeps him moving. He no longer trusts Banquo. So that one set of prophecies continues to push him along.
He chid the sisters,
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown (Act 3, Scene 1)
Macbeth decides that Banquo has to go, and kills him.
After the banquet, Macbeth gets antsy. He decides that Macduff is also a threat to him. He plans to kill him, but when he is not there he has the entire family killed.
The second set of prophecies also cause trouble. Hecate ups the ante, making Macbeth even more a puppet by making him think he is invincible. She tells him that he cannot be killed by man born of woman, and that he is safe until the forest comes to Inverness. Naturally he thinks he is fine. All men come from women, and forests can’ move! Yet these prophecies are deceptive.
Though Birnam Wood be come to Dunsinane,(35)
And thou opposed, being of no woman born,
Yet I will try the last. (Act 5, Scene 8)
The predictions come true, and Macbeth is defeated by Macduff. All because the witches made him want to be king, fear to stay there, and then think that he was safe until he realized he was not and gave up.
The witches add an important mood to the play. They make it darker, and add elements of magic. These make the floating daggers and ghosts fit in. The witches also allow Macbeth to make the transformation. They serve as catalysts to turn him from a loyal hero to a bloody tyrant.
Weired sisters start Macbeth on his way to becoming a king by playing to his ambitions when stating "All hail Macbeth! Hail to thee, Thane of Cawdor! All hail Macbeth, that shalt be the king hereafter!" (1.3.52-53). Through the story they manipulate him by predicting his future as a great king, and finally push him to his tragic demise. By their predictions, they give him a false sense of security in his future.