Sometimes the songs are adapted from another language, and the chorus for example is left untranslated and after a generation or two becomes garbled into nonsense syllables - The many incarnations of Shule Aroon for example. Another example from African American tradition: the words "Lordy Lord" get corrupted to la-dee-da etc.

Also, along with what tradman said about drums and other instruments being taught with special syllables, I've read that during the time bagpipes were banned in Scotland (or was it harps in Ireland?), this is the way the music was preserved in the oral tradition. It's not so different than us using do-re-mi-fa-so-la-ti-do to learn to sing.