In normal speech, vocables perform various functions - for instance, 'um' and 'ah' buy us time to think, and paper over cracks in our phrases; and babies testing out their vocal cords tend to be pretty keen on the vocables. Not sure they have a wealth of alternatives at that stage, to be honest. So in speech, vocables aren’t meaningful, or consequential, or even intentional - but in song, they can be all these things. All those la la las and dum di dums and bom bom do be de doos are ubiquitous in songs - so what are they doing there?