In Macbeth, why are the three witches important?

In William
Shalespeare's play , the three witches are very important. They set the
mood, set the plot, and to bring the supernatural into the play. 

First,set
the mood of the entire play. Given the witches are seen in the opening scene of the first act,
the mood is defined as ominous and dark.

Second, the witches are important
for their actual role in the play. The witches are responsible for providingwith the
prophecies....