This scene accomplishes two things. In it we meet the head of the witches, Hecate, and it also reinforces the witchcraft idea.
Hecate it the boss, so to speak, and she is none too happy with the three witches who have met with Macbeth. She calls him a wayward son and tells them they should not have told him anything. "Spiteful and wrathful , who, as others do,/Loves for his own end, not for you."
To get back into her good graces, she instructs them on just what she wants them to do the next time they meet with him. They are to tell him and show him certain things which will lead him to think that he cannot be beaten in the war he knows is coming. This, of course, sets up his over-confidence which leads to his downfall.
The second half of the scene with the sprites shows Hecate's power but it also plays into the Elizabethan audience's belief in witchcraft. Witches and demons were very real to Elizabethan England. King James himself had written about witchcraft.