In Macbeth, what is important about the witches' prophecy?

The weird sisters' prophecy has several important functions in the play. First, their meeting on the blasted heath creates an ominous atmosphere of supernatural horror at the start of the play. It is one of the most dramatic scenes in the play and an audience favorite. 

More importantly, Shakespeare lived in a place and period where most of his audience would have been Christian and believed witches to be evil and allied with the Devil....

The weird sisters' prophecy has several important functions in the play. First, their meeting on the blasted heath creates an ominous atmosphere of supernatural horror at the start of the play. It is one of the most dramatic scenes in the play and an audience favorite. 

More importantly, Shakespeare lived in a place and period where most of his audience would have been Christian and believed witches to be evil and allied with the Devil. Thus they provide a religious and moral context for understanding the play.

Next, they set in motion the main plot of the play. It is their prophecy that causes Macbeth to turn his ambition, which has previously been manifested in loyal service to Duncan, to focusing on killing Duncan and taking the throne himself. Because it is evil witches who tempt him to do this, the audience knows that his choices are not justified and that he will prove evil as well. The witches constantly bring out the worst in Macbeth.