The witches serve as catalysts in the opening act of Shakespeare's Macbeth. They initiate the conflict that leads to the rising action of the play.
They know ahead of time who's going to win the battle, they know where Macbeth will be after the battle, and when they meet with him they tell him he will be Cawdor and king. When Macbeth finds out that he has, indeed, been named Cawdor, his natural instinct is to take steps to secure the thrown for himself.
Of course, not everyone agrees the witches are supernatural. Some commentators cite evidence that suggests they are just very smart and manipulative. For instance, Duncan names Macbeth as the Thane of Cawdor before the witches make their prediction to Macbeth. There's not necessarily anything supernatural about the witches knowing this.
Regardless, though, of whether or not you see the witches as supernatural, they still serve as catalysts in the play.
The supernatural (the witches) play a huge role in this act.
First, we see them in Scene 1 setting the tone for the play by telling us that evil things are going to happen. But their main impact comes from the things that they say to Macbeth. When the witches see Macbeth after the battle, they tell him that he is going to be Thane of Cawdor and King of Scotland.
When Macbeth hears this (and then when he finds out he has been made Thane of Cawdor), the ambitious side of his nature perks up, which leads to his eventual downfall. He starts to think about killing Duncan so that he, Macbeth, can become king.
In the play 'Macbeth' by William Shakespeare, the author adds a whole new dimension to the early English works used by playwrights and writers of that period to base their historical entertainments upon. Always popular, there was a danger they might be a little 'dry.' William Shakespeare, in a stroke of genius, adds a touch of the paranormal to 'jazz it up a little!' King James was known to have an interest and a superstition about witches and so did many other people,sadly. So it was a doubly clever piece of inspiration. The witches and the supernatural also gave an excuse for clever machinations and effects on stage and even now they have a role keeping us interested!