Nonsense Sounds Help Build Language Skills
Phonological Awareness, or the ability to discern the sounds that make up words, is one of the first skills that children learn - hence why songs, rhymes, and words in general are such positive inclusions in storytime. Separating the sounds in words, as is done in songs and nonsense rhymes, helps to solidify the individual sounds that make up a language and promotes improved speech.
Many of the children that attend my storytimes (and yours, as well, I would guess) probably speak a language other than English or hear a language other than English on a regular basis at home. Breaking apart words into their component syllables through fun songs helps children to discern between the nuances that exist in words from any language. The more experienced children are at listening to the sounds of words, the clearer and more adept they will be at speaking not only English, but any language they might be picking up at home!