Why did Shakespeare decide on having another meeting with the witches in Act 4 and with more predictions?In Macbeth there was one meeting in Act 1 and a 2nd in Act 4.
The three witches are very effective dramatically whenever they appear in the play. Shakespeare was not only a poet and a playwright but a producer and director. Once he had created these Weird Sisters and employed three actors to portray them, along with providing the appropriate costuming and stage props, he undoubtedly wanted to get the maximum emotional effect out of them (as well as the maximum value for the money he had to pay them). Their first appearance is very brief and is meant to shock the audience into silence and attention. The second appearance is also brief and serves mainly to give Macbeth something to think about and to report to his wife. In Act IV, Scene 1, Shakespeare puts on a full-scale display of his witches and the apparitions they evoke. It makes good theater. It is good stagecraft. It frightens the audience. It is something they will remember after they leave the theater. Shakespeare was well aware that plays present visual spectacles as well as characters spouting dialogue. He must have gone to considerable trouble arranging for the appearance of the witches and all those apparitions. The stage directions in the text cannot give an adequate idea of what the scene must have looked like. Naturally, the scene also serves to provide information. It reinforces Macbeth's motivation and also tells the audience that Macbeth will be a tough man to overcome if Birnam Wood has to move to Dunsinane and if he cannot be defeated by any man born of a woman. Shakespeare's audience did not know, as we do, that Macduff was born of a cesarean operation.