It's such a small feature but imagine having a song buffer instead of playing it directly from your phone memory.
You have to log out the app, then press another one search for that one track you want to listen to, and what if you want to listen to a song you haven't downloaded yet, you repeat the process just to get to another app.
That changes now with the new features of YouTube Music which has been regarded as the full replacement for Google Play Music with the little option of playing straight from your phone's local collection.
There are limits, at least in the current iteration. You can't add local tracks to playlists or queues that include YouTube Music songs, and you can't cast local songs elsewhere. Not surprisingly, the playback controls also remove YouTube-specific features such as the like and unlike buttons. If those aren't deal-breakers, though, you now have a way to embrace Google's current vision for music without quite so many compromises.