Why does Macbeth go to the witches and what do the new prophecies teach Macbeth? Act IV

In Act IV, Macbeth goes to the wtches demanding answers to his pressing questions about the future. The witches complete their magic spell and summon forth a series of apparitions. The first is an armed head that warns Macbeth to beware the Thane of Fife (Macduff). The second apparition is a bloody child, who tells him that "none of woman born / Shall harm Macbeth" (96-97). This news bolsters Macbeth spirits. The third apparition is a crowned child with a tree in its hand, who says that "Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him" (107-09). This cheers Macbeth even more, since he knows that nothing can move a forest. Macbeth proceeds to ask his last question: will Banquo's children ever rule Scotland?

The cauldron sinks and a strange sound is heard. The witches now show Macbeth a procession of kings, the eighth of whom holds a mirror in his hand, followed by Banquo. As Banquo points at this line of kings, Macbeth realizes that they are indeed his family line. After the witches dance and disappear, Lennox enters with the news that Macduff has fled to England. Macbeth resolves that he will henceforth act immediately on his ambitions: the first step will be to seize Fife and kill Macduff's wife and children.