When we first see the witches in Macbeth, they are planning to meet Macbeth on the heath. Why do they want to meet him?

The witches want to meet with Macbeth in order to plant the seed of kingship in a brain already full of ambition.  The witches are not definite in their reasoning the first time the reader hears them speak; however, they give a clue as to their purpose in the following lines:  "Fair is foul, and foul is fair. / Hover through the fog and filthy air" (1.1.10-11).  Macbeth (we assume) has never had the gall enough to usurp the throne from Duncan.  Therefore, when the witches arrive and make a prediction the question arises whether they are putting a new idea in his head or whether they are simply fueling the fire of Macbeth's ambition making it that very "vaulting ambition" that Macbeth has become so famous for.  In my opinion, it can be true and supported through evidence either way.  That is the beauty and the ambiguity of Shakespeare's genius.