The practice of replacing actual words with meaningless words in songs continues with abandon, and for good reason:
Without meaningless words we would never have had many of the greatest anthems in pop. … [L]yrics without meaning often appear in songs that might, in some circles, be described as “catchy”.
One explanation for this is that catchy songs are catchy because they’re easy to sing along to. Meaningless words help that process. It’s also true that catchy songs often have such phenomenal hooks that actual, meaningful lyrics would struggle to keep up with the tune.
Next time “na na, na-na-na-na” is playing over and over in your head, you now know why. See also.