I tried an experiment. Try the songs you know with these bits in them and instead of singing them, play the notes on a guitar, flute, fiddle whatever instrument you have?

I think musically a lot of the songs contain phrases that are questions. The melody phrase is incomplete to our ears without the answering phrase.

So the melody has to be taken back to enable a new musical question to be asked. It is more natuarally done instrumentally, but in the absence of these, historically or many reasons (including legal bans, it would be done by singing the line, fol de riddle tiddle dum day?