What important role do the witches play in Act I of The Tragedy of Macbeth?

In Act I of the Tragedy of Macbeth, we are introduced to the "weird sisters", which are witches. They are described as beings with ties to the "other world", with powers to affect people's lives and even the weather at will.

They represent Macbeth's tragic destiny, they foretell what will become of him: he will rise to be a king, but they also prophesy his demise, but that will become clear further in the play.