Somewhat more common in novels than in RPGs, but... A lot of times, there is a Prophecy. Now, obviously, you have to accept a certain sort of magic in the world for there to be prophecies at all, but you really have to wonder about the gods in books where prophecies exist only in very bad rhyming verse. Sometimes even the characters in the book admit that the rhyme sucks (signal from fred!) but the question of WHY it's rhyming is rarely addressed.
Surely, if the ancient gods wanted to give you a poem, they could make it a poem that didn't suck. Divine powers and all that.
Now, there would be reason within a game world for someone to turn a non-rhyming prophecy into a rhyming one, or particularly into a song. It's easier to pass down the oral tradition that way. Which can be handy in fantasy-medieval communities where literacy isn't all that common.
It would be nice to see a plot in which the characters had to track down the original source of a prophecy and find out just how distorted it had gotten over time through the license of bards...