"By the pricking of my thumbs, something wicked this way comes"
-the witches predict macbeths arrival before the audience has any knowing of his becoming, sensing the evil and corruption of macbeth reinforces their supernatural connections to fate, and reinforces the darkness of their omniscience, they know macbeth is coming, perhaps reinforcing his fate has been decided
-with macbeths fate decided, and being judged by even the supernatural as "wicked", shows macbeths downfall is solidified and complete, he has fallen for their deceit, the murder of duncan has occured. shows how the supernatural corrupt and destroy original forces of good by praying on human weaknesses, his ambition was picked a part and toyed with by the witches.
-a deeply superstitious society would have resonated deeply with this supernatural change, fearing deeply its consequence on macbeth, this line in particular stand out, in Iambic Pentameter