Why do ballads include repetition?

Back when very few people knew how to read, balladeers and troubadours would travel around, passing on the traditions and history of the world through poems and songs. The use of repetition and rhythm made it easier to remember the songs (some of which were enormous, like Beowulf). The tradition stuck: repetition is used to emphasize the song's theme, and makes is easier to remember the song. For those who prefer to listen to the music rather than the lyrics, repeating various points of the song gives cohesion to the song.