Our knowledge of folk music goes back a few centuries. In that time, thousands of people have sung them to many, many tunes. Given that, there are going to be a lot of different reasons for nonsense rhymes.

Good reasons have been given so far - to imitate foreign language, to imitate an instrument, to provide an easy chorus. Another reason is that a pre-exising tune is longer than the new lyrics, so nonsense syllables are used to fill it out.

And finally, let's not forget that it's fun to do.