Does my song need to have been released commercially on iTunes or other platforms?

Not necessarily, since ads will be placed on the videos using your songs anyway.

However, in order to maximize the revenues and generate additional downloads, on top of the ad revenue you may get from YouTube, it is recommended to release your song on iTunes and GooglePlay with our service so that the download links are automatically added under the video description.

This feature is part of the monetization process, where YouTube matches on the song's ISRC and inserts stores' download links under the video description.