Convenience. I have like 150 gigabytes of music. I can't put all of that on my phone and even if I just put a subset on there I'd have to re-load it every time I get a new phone. What if I want to play the music out of whatever laptop or tablet I happen to be using at the time? Also, I don't have to worry about my hard drive dying. Also, Google lets you save the music to your phone so you don't have to use mobile data.