Spies have significant benefits over Diplomats: first, they have 3 movement points instead of 2, so they can move faster. In addition, the preexisting function that Diplomats are able to do Spies do better: Spies may establish embassies or investigate the city for just 1/3 of a movement point, can survive an attempt at stealing technology or industrial sabotage with bonuses, and lastly can directly harm the opponent by sabotaging units, poisoning water supplies or planting a nuclear device.