Why Helen known as 'the face that launched a thousand ships'?

When the goddess of strife and discord, Eris, presented a golden Apple "to the fairest" at Thetis's wedding to Peleus, Athena, Hera and Aphrodite got into a big disagreement. Zeus declared that Paris, a Trojan prince (he didn't know he was a prince since his parents left him in the mountains when the received a prophecy that he would destroy the city. He lived as a shepard), would decide. Paris's verdict was that Aphrodite was the prettiest so in return, she granted him the most beautiful woman in all of Greece. That just so happened to be Menelaus's wife, Helen of Sparta. Paris "kidnapped" Helen and brought her to Troy to be his wife. Sparta allied with Mycenae (Menelaus's brother was Agamemnon of Mycenae) and waged war with Troy. Troy fell and Menelaus got Helen back. In short, a huge war was fought, a whole city fell and many people died just because of a beautiful face.