E-ink screen is easier on the eyes, and also helps the battery to last for way longer than a phone would. Some people also appreciate having the compartmentalisation of a separate device that's just for reading — fewer distractions.

My phone is pretty heavy, and reading on it can hurt my hand after a while, especially as there isn't an ergonomic way to turn pages (the app I use can have it so the volume buttons turn the page, but they're not positioned well to do that).