Why did Jason and the Argonauts need to retrieve the Golden Fleece?

King Pelias, of the city of Iolcus in Hellas, sends Jason and the Argonauts up the straits into the Black Sea in search of the Golden Fleece. Pelias is compelled to do so because of a prophecy that haunts him. An oracle warned him that a man appearing before him with one shoe would herald his death. Jason, who lost a shoe in the muddy bank of a river, appears before the King confirming his fears about the prophecy. Hoping that he will never see Jason again, he sends him off to sea.