And what exactly is the point of "advertising" a song to people who, for all intents and purposes, already own it?

To get fans who will buy records, concert tickets and merchandise.

There must be some point, or otherwise it wouldn't be done. Follow the money.

Care to provide some evidence that it is being done, ie. that artists are happy with the payment arrangement negotiated on their behalf with Spotify et al on the basis that it constitutes cheap advertising?

I think he's referring to radio.