Shakespeare's Macbeth is said to have been written in honor of James I (of England, who was also James VI of Scotland). After Elizabeth I died without an heir, the English throne passed on to James, her cousin. James was Shakespeare's patron—having chosen Shakespeare's acting company to be the King's Men (his own actors), quite an honor—with regular pay. The Bard did his research, writing about Banquo (James' ancestor), presenting him as a valiant fighter and honorable man. This aspect of the play would have appealed to James. The elements of the supernatural in the play are present for several reasons. Interest in the supernatural did not begin with James, but he was very, very interested in it. In fact, he wrote a book called Daemonologie, and was very concerned with the supernatural—however, his interest was more of a religious nature. James believed:
Thou shalt not suffer a witch to live. (Exodus 22:18, KJV)
(This verse comes from the Bible, the King James Version, which James commissioned to be written in a more poetic form.)
James was interested in ferreting out witches and punishing them.
As with Shakespeare’s plays, James identifies different motives that make people give in to the temptation of these ‘agents of the devil.'
Shakespeare would have introduced the supernatural into the play because James was interested. However, Elizabethans believed completely in the presence of ghosts, witches, fairies, etc. A wise playwright, Shakespeare included elements of the supernatural in his plays that would appeal to a wide audience.
Witches are also believed to be old hags who are 'thought to be in league with Satan' and who possess strange powers of darkness given by the dark lord...[able to] ...cause storms, diseases in animals, self-invisibility, and [flight] through the air.
The witches in Macbeth are unquestionably evil. Their predictions get Macbeth to think about becoming King of Scotland. They lie to him to trick him into committing a mortal sin: the killing of a King. They tell Macbeth half-truths:
Macbeth shall never vanquish’d be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (IV.i.103-105)
To Macbeth, this sounds as if the woods would have to move onto Dunsinane Hill. However, Macbeth is beaten when it looks like the woods are moving—because the soldiers have camouflaged themselves. Malcolm gives the following instructions:
Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (V.iv.6-9)
Malcolm attempts to hide his numbers from Macbeth, so the men shield themselves with branches, making it look as if the woods are moving.
Hecate, Queen of the Witches, grows angry with the Weird Sisters because they have been playing with Macbeth, and they haven't included her so she can enjoy the fun. However, she wants to destroy Macbeth and rob him of his immortal soul. After all, she serves Satan, and none of the audience would expect anything less of her character.
Much the way we may go to scary movies to be frightened, so, too, the Elizabethans went to the theater—for they believed witches walked the earth, trying to harm humans. The audience believed everything they had ever heard of witches, and other supernatural creatures.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.