During the banquet in Act III, Scene IV, Macbeth learns that Banquo has been killed (as he ordered) but that Fleance, Banquo's son, has escaped. Remember that back in Act I, Scene III, the witches made three prophecies and that the last of these stated that Banquo's sons would rise to become kings. Macbeth is, therefore, very worried that Banquo's son, Fleance, will overthrow him and become the king of Scotland. At the banquet, Macbeth expresses this fear through the following lines:
"Then comes my fit again. I had else been perfect."
For Macbeth, then, seeing the witches again is important because it will help him to determine whether Fleance is indeed a threat to his throne. After all, everything that the witches told Macbeth has so far come true: he became the Thane of Cawdor and then the King of Scotland. Now he needs to know if Fleance will really take his kingdom and if any other threats may cross his path.