why this device works so well
Figure of balance in which two contrasting ideas are intentionally juxtaposed, usually through parallel structure.
Antithesis is a very effective rhetorical device, it contrasts two ideas to make a point and give your speech rhythm.
1. "It was the best of times, it was the worst of times" Charles Dickens, Tale of two cities
2. "One small step for a man, one great leap for man kind" Neil Armstrong, Apollo 11
3. " Time is money, but money can't buy time"
Neil Armstrong One Small Step For Man