The witches have chosen the right moment to approach Macbeth - when he is full of the triumph of battle, and fresh from killing.
Macbeth begs the witches to stay and tries to question them.
Stay, you imperfect speakers, tell me more."Macbeth (Act one, Scene three, Line 70)
It makes you wonder whether he's already thought about what they are saying, ie about being king. And what are those 'horrible imaginings'(Act one, Scene three, Line 138) he then speaks of? Could they be of Duncan's murder? The witches could only have so much power over Macbeth if he already had these thoughts in his mind.
The witches speak Macbeth's innermost thoughts. They know exactly what to say, and their timing is precise. They seem unnaturally close and act with one mind.
Then Ross and Angus arrive with the news we already know - Duncan has made Macbeth Thane of Cawdor.
He bade me, from him, call thee Thane of Cawdor."Macbeth (Act one, Scene three, Line 104)
The first prophecy has come true. Macbeth has immediate proof the witches know the future.
They've got him - Macbeth is under the witches' spell. From now on, their words will always be in his thoughts. We only see them once more, but the witches are always in Macbeth's mind.
When the consequences of the murders of Duncan and Banquo are too much for Macbeth to handle, his instinct is to go back to the witches (Act four, Scene one). They are now his only hope. It is an admission he cannot control things any longer and from this point in the play, we know Macbeth will die.