Basically it is said that if Persia had managed to conquer Greece, then much of Greek culture would have been suppressed or even lost forever.
Much of Western thought comes from the Greeks, so such ideas as Democracy and what not may have been lost forever.
It showed the west and the whole world that no one operates outside the bounds of Almighty God.
All the events of the Persian and Grecian empires were predicted in Daniel 11 long before they happened.
