The witches set the tone for the entire play as dark and supernatural. This also somewhat removes the culpability from Macbeth's hands, while tying the supernatural evil to the other main woman in the play, Lady Macbeth.
They advance the play by informing Macbeth that he is destined to be King. If he had not been told this, he could not have told his wife, and she could not have taken it as a sign to kill Duncan.
However, the witches acted without permission from Hecate, the witch queen, and as punishment must act again later in the play to sabotage the king they have made.
It is noteworthy that many modern Macbeth productions actually remove the three witches altogether, preferring to leave at least some of the responsibility at Macbeth's feet, as well as remove the supernatural aspect which they feel conflicts with the rest of the play's more visceral themes.
The witches serve a number of purposes. In general, Shakespeare is setting up a good vs. evil type of scenario. He wants us to recognize this is a different type of world where the powers of evil definately have some control. But it is also important to recognize in some of the foreshadowing, that man always has the ultimate choice; evil has no power unless it is allowed to. Beyond that, the original scene also adds to the tone and mood of the play. You get a feeling from the very beginning of the dark nature of the play.