I recently discovered that videos that you buy on play store can be viewed on your you tube player (if you are logged in) This is good because YouTube is everywhere. I noticed that some smart boxes won't play your purchased video. I think it is something to do with the box not informing youtube of its location, this seems to be a bug. It is nice to chromecast your purchased play store vid collection on everything.
The Chromecast on YouTube is different to the Chromecast of other apps because it works with non Chromecast devices such as smart tv boxes and games consols.