Ever since YouTube released YouTube Music in May, users have been less than pleased with the experience. The large collection of music has not been enough to compensate for the terrible user experience or the complete lack of common music streaming service features. Many users have been so disenchanted that they switched back to services like Spotify.
The company has finally recognized their own shortcomings and has put into place a process for releasing new features to the platform. Starting now, YouTube will release new features to their apps every 2 weeks, with external storage releasing now. That means that, with the most recent release, YouTube Music can store music to an SD card or hard drive, which is good for most users who have a small amount of internal storage and a large amount of external, specifically for media.
In addition to feature additions, the company plans to fix a number of the interface issues. Hopefully, these changes will include making music searching more useful, as well as album discovery and content sorting. All of these things are standard fare for streaming services but are all made painful in the YouTube Music platform.
Of course, it's important to remember that YouTube Music is still very new, only being released less than 3 months ago. It is unusual for Google to get any interface correct in the first release (in some cases people don't even know what the product does). They do usually figure it out with time, which is likely how this product will go, as well - in time.