I would say that to put it very reductively, the "dramatic
significance" is "be sure your sins will find you out."
Macbeth and Lady Macbeth are consumed with fear and guilt, both
propelled by their insatiable desire for power.
As for the witches, they are yet another warning for Macbeth
that his sins will not go undiscovered or unpunished. They
prophesy his fate. He does not want to believe them, but they
speak a bloody truth. The first witch tells him "to beware
Macduff." The second promises that "none of woman born shall
harm Macbeth." The third tells him that he will be safe until
"Birnam Wood moves against him."
The witches prophecies add to the drama because as an audience,
we know these things will come to pass, even if Macbeth and his
Lady choose to stick their heads in the sand. It is just a
matter of time until each of the prophecies come to pass.