The main reason for repeat lines is typified in Breton music, it's so the song is reinforced in those who know it and learned line by line by those who don't. It's the core of oral transmission. Anyone teaching by ear does exactly the same, mostly they run through it once to give the shape, again to teach the detail, and a third time to hold the learners' hand. Then a few times to perfect it. A few more to play with it to make it your own and find the balance against the others doing the same. And suddenly it's not quite the same as taught. Folk is at work.