Earlier quoted context omitted.
It's free with Prime and cheaper than other alternates for the paid version if you have Prime. For casual listening it's not bad at all. It also supports so called HD and Ultra HD, which I believe Spotify still doesn't support, while Apple does.
Does Apple or Spotify have some sort of extra feature over Amazon that would appeal to non-casual listeners? I had Apple for a bit, switched over to Amazon because of the Prime discount... while I'm aware that morally I should de-Amazon my life, the service is pretty convenient.
For quite a while, Google Music wouldn't work on Echo devices and Amazon Music wouldn't work on Google Home. Spotify worked everywhere, had native applications, and a decent value for a family plan.