• Separate batteries. Using a device for music and a standard phone drains from the same battery. You could carry a power brick, but then you’re carrying two bricks for worse audio.
• No camera. Certain work assignments won’t allow me to bring a device with a camera into those zones. Or, if I do, the transition process is so intrusive that it’s not worth it.

Those are the only unique characteristics. You can compensate other differences on a phone like adding an additional DAC and/or amp.