I think the categories are specious, because any song that doesn't fit in a familiar slot (love, comfort, friendship) can be stuck into 'songs of knowledge,' a category so broad as to be undefinable.

A song about a ghost? 'I know a ghost story.'
A song about work? 'Here's how to do this job.'
A song about radishes? Teaching the next generation life skills.


Don't laugh. I had a roommate once who used to sing a song in Spanish about radishes.