There are lots of references to blood, evil and the night. These are all designed to create atmosphere. Then there is Lady Macbeth. When she calls upon the spirits to 'unsex her' so that she can plot the murder, this would create suspense in the play. Then Macbeth nearly decides not to do the murder. The way his wife puts him under pressure, creates futher tension. She has already said that he is too kind to do the deed. If he doesn't then all will be well for him. So it is a matter of tension when she is persuading him to do evil.
Before he does the murder there is the strange scene with the dagger, and again as he makes his way to do the killing, he talks about night and evil creatures stalking the world. During the murder scene it is very tense as Lady Macbeth thinks thye have been caught. Then Macbeth returns and they talk in quick, short snatches (staccato- a term from music which expresses the way they talk as he descends from doing the killing). Then there is the whole business of the knocking at the door and Macbeth beginning to lose his grip. All these techniques help to create the intense atmosphere of evil, doom and suspense.
The witches in 'Macbeth' serve a variety of purposes: Their most obvious role is to give Macbeth big ideas about his future and to create ambitions within him. They also make him believe that he is invincible:'none of woman born / Shall harm Macbeth' It is because of this belief that Macbeth is willing to take ever- greater risks. The witches help to chart the changes that occur in Macbeth's character. Look at how they refer to him each time he arrives in their presence (by the end they refer to him as 'something evil').
Macbeth's relationship with the witches also 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 important. Macbeth 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. The witches help a great deal in the creation of an atmosphere of evil and a sense of events running out of control.
