Why is the first scene of Macbeth important?

The first scene of Macbeth is
important because it sets the tone of events to come.  In the scene, the three witches
meet and discuss when they will meet again.  Here, the reader learns that a battle will
ensue and that they will come together afterwards.  But they say that the battle will be
both lost and won which is a paradox meaning literally that the battle will be lost by
one side and won by the other.  Figuratively, the statement refers to Macbeth's
future--he will win the "battle" of becoming the King and reaching his ambitious goals;
however, he will lose his family, the respect of his followers, and eventually his life
in the process.  Further, the witches end the scene with another paradox:  "Fair is foul
and foul is fair."  This sets the tone for the play--all will not be as it
seems.