Macbeth needs to know more information about the future. So far everything that they told him has come true. He was Thane of Glamis, he became Thane of Cawdor, and now he is the king. However, when Banquo's ghost appears--and only Macbeth can see it--he remembers the prophesy that Banquo's sons will be kings as well. Macbeth needs to know what is coming so he can prepare for either Fleance, or to see what is in his future. He is becoming a killing machine at this point, and his famous line to his wife in Act III, Scene IV is "It will have blood; they say, blood will have blood." Macbeth is very concerned that he will lose his crown and he wants to make sure he knows what is to come, which is why he goes again to the witches.
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.
according to me in act 3 scene 5 he (macbeth) wanted to meet them for getting some information about his fiture what is going to happen with him.and in act 1 scene 2 when they(macbeth and banquo)mate the 3 witches the thing which they had said they all came true like they said thathe was thane of glamis then he will become thane of cowder,and then the best one the king of scotland these all come true so for this reason he thought it will be good for him if he ask about his future to the witches