
Opening with the witches is an effective device because it sets the mood to be eerie and unnerving, similar to how the ghost functions at the beginning of Hamlet. Both of these supernatural phenomenon prophetically appeal to the titular character in order to drive the plot of the play.
However, the witches are in some ways less mysterious than the ghost of King Hamlet, because early in the first act their prophecies are confirmed to at least hold grains of truth, revealed first to the audience and then to the characters of the play. It is significant that Macbeth's first words of the play "So foul and fair a day I have not seen" echo back to the witches' line in scene 1:"Fair is foul, and foul is fair." This immediate correlation between Macbeth and the witches indicates the power of their prophecies and anticipates the power they will have over Macbeth. Macbeth is much more stricken by the words of the witches than his friend Banquo, who begs to be included in the witches' predictions but does not feel as consumed by them as Macbeth.
Shakespeare's use of paradox and prophecy together is a brilliant literary combination of techniques, because it entices the subjects (or perhaps victims) into obsessing over their respective futures, trying to accommodate the details of their lives to those described by the witches. Needless to say, it won't end well for anyone, because, you know, that's what happens in tragedies and also whenever witches or prophecies are involved.