What is the role of the witches in Macbeth and the atmosphere they create?

Putting the play into perspective, Shakespeare's audiences
believed in the supernatural and witchcraft.  The king himself was interested in the
subject.  For playwright, William Shakespeare, it was a natural structural
element.

Who are these women?  They are called the weird
sisters, weird coming from an Anglo Saxon word which meant fate.  In this sense, they do
not "make" things happen, they just function as a kind of oracle.  They tell Macbeth
that he will be king in the future.  They do not tell him to kill
Duncan.

When Macbeth attempts to kill both Banquo and his
son, Fleance, he has interfered with fate and therefore must pay the price.  He is led
to believe that he is invincible but he is not.

Exactly who
these women are is up to interpretation.  Their function is to create a supernatural
atmosphere where "nothing is but what is not".