Mostly power. Phones just don't offer enough to drive demanding headphones or earphones. There are dongles to address this issue, but they can drain the batter

Mostly power. Phones just don't offer enough to drive demanding headphones or earphones. There are dongles to address this issue, but they can drain the battery pretty fast. I don't want to drain my phone battery just because I'm listening to music. If you have an iOS device, having offline music has plenty of limitations. It takes a ton of space that I'd rather save for pictures, videos and apps. And you have to sync your phone with a MacBook which is a hassle if you have more than one.