There's no desktop app for YouTube Music for starters.
I can't think of a single reason I'd want/need a standalone app over having the Chrome version of the app, which to all intents and purposes appears as a standalone app anyway.
So I'm curious, what's the use-case for a Desktop App to stream music? Even with the webapp you can download music for offline play.
FYI you can also do this with Spotify[0].
Local files work fine if you're always playing music on devices you own and that have local storage. But if you're using media devices like a Chromecast (unless you're casting directly from a device that has access to the local files), or on machines where you don’t have sync privileges—like a work computer—YouTube Music will work, but Spotify won’t.