Also, in the poem, there's a theme of how the father not believing his son that the Erlkoenig is real leads to the son dying/being taken by the Erkloenig--ie 'believe me when I talk about the world or you'll die horribly too, don't be a cinderella'.

Opposite's communicated a lot with poetry before--not a whole lot of free people actually study it, it has a very high information density, and it's a good way to simultaneously communicate complex emotional and philosophical information to other educated slaves *and* impress the free people with his good memory and intelligence and education. He's had whole long, complex conversations entirely made of him and the other people quoting poetry at each other. It's an oblique way of communicating, but popular among rich-people-slaves.