Your choice of,
comfortable to hold in your hands captures part of one major reason, but let me expand on it.
- I can set it down and continue reading while my hands attend to some mindless chore.
- I can let go of it for a second or two without losing my page.
- I can hold the whole surface sideways so I can read with my head on the pillow whereas with paper I can only comfortably do that with left pages.
- I can continue reading as I get up from my chair and start walking (easier than with paper).