He opens with the witches to give the play an ominous feeling. At the time Macbeth was written, witchcraft was heavily associated with evil and the devil. So, by opening with the witches, Shakespeare sets the tone for the whole play.
And, although Macbeth doesn't appear, we are told of his great heroics in Act 1, Scene 2. This creates the image of a strong, valiant warrior, and we expect to meet him in such a light. However, since the witches said that they will meet Macbeth, we associate him with them, and assume that, like the witches, he is not all he appears to be.
