What is the significance of the scene between herrick and the accused witches?

The witches appeared in the opening scene, in Act 1 Scene 1, for the purpose of laying the foundation of the plot. The stage directions calling for thunder and lightning were there to heighten the dramatic effect, and especially to increase the feeling of terror that the witches themselves brought with them. Indeed, the entire play could be considered basically a story of terror, with the terror being relieved only after Macduff's appearance in the last scene, in Act 5 Scene 9, with the severed head of Macbeth [c. 1014-August 15, 1057].