It's a mental thing. The phone is the doorway to all the things, and the reader is the doorway to reading. When I'm being intentional about my phone usage (not always, but most of the time), I defer to devices that are focused.
And the phone's reader app?
But then I'm subject to the bright screen, all of the colors, all of the notifications, and everything else that the phone brings. The calm of the X4 is quite nice.