so helps the reader to see the changes in him, initially they find him, by the end he is calling to them. You could also look at how he speaks to them; he gets more and more familiar and even rude as he gets further away from the usual constraints of human society.Finally, on a more practical level, they literally provide a change of scene and of tempo to keep the audience interested. People in Shakespeare's time would view them quite differently to a modern audience because most people believed in witches. You ask if they are to blame for what happens to Macbeth. He is the one who listens to and acts on their advice so he is partly to blame for his own downfall. Lady Macbeth really encourages Macbeth to kill Duncan and she actively helps him to cover his tracks so she is partly responsible for starting him on the path of evil too....
