For me, having a local music library is doubly as important because I am often out of reliable cell service. I usually rip CDs to my iPhone, but as of yet I have avoided paying for any music services. I generally prefer to own my music rather than rely on pulling it down from somebody's server, not to mention that streaming audio uses significant battery on a mobile device.

I use YT music when I have reliable internet and I want to listen to something I don't own.