The problem with a Music is it is trying to make you use the streaming end of the app, or at the very least, see no difference between streaming music and local content. Local storage is less clear to access. This means the user gets confused.
This would be fine if we all had unlimited or data caps so huge we would never use them, but we don't.
And then there are the poor sods with business phone where the business doesnt allow/provide for streaming non work data.
The only option away from home wifi is locally stored music.