What is the significance of having the apparitions give the info to Macbeth?

There are two scenes in which the witches deliver prophecies to Macbeth (in Act I and Act IV), and in both cases his future actions and their outcomes are guided by what he learns from them. By having the events of the play set in motion by a supernatural source, Shakespeare invites us to question, from the outset, the uncertain relationship between predestination and choice. There is no indication that, prior to meeting the witches, Macbeth harbors any expectation or ambition of becoming king, and he certainly has not formed anything even close to a definite intention of murdering Duncan. The irony is that the witches' first predictions are a self-fulfilling prophecy; once the idea, the possibility, the desire, the fear of what he himself might be capable of, are planted in Macbeth's mind, they swiftly grow into an inevitability. This applies not only to the murder of Duncan but also, in a doubly ironic sense, to the future reign of Banquo's heirs. It is Macbeth's fear of being succeeded by Banquo's descendants which motivates his murderous paranoia, and he fears this precisely because, after all, the witches' first prediction did come to pass—through his own actions. In attempting to defy prophecy by ordering the deaths of Banquo and Fleance, Macbeth instead ensures that he will be deposed. The witches' "supernatural soliciting" thus introduces a chicken-or-the-egg question which haunts the play: events transpire as foretold because of Macbeth's actions, which are motivated by what the witches have told him will happen. The prophecy comes true because Macbeth believes it.

All of this is complicated, however, by the second encounter with the witches, in which they damn him further by providing him with deliberately deceptive information. This later scene makes it clear that the witches possess genuinely paranormal insight, but continues to obscure the already tenuous relationship between perception and reality. In this instance, Macbeth (mis)interprets the new predictions he receives in the most obvious advantageous way. In the first encounter, his knowledge of the future shapes his desires; in the second, his desires misleadingly shape his understanding of what he is told about the future, consigning him to even greater despair when those prophecies prove to mean something other than what he has assumed. In perhaps the broadest sense, the presence of the witches creates an extraordinarily dark portrait of humankind at the mercy of hostile forces beyond our comprehension, and implies the ultimate futility of human choice.