At the risk of making it sound like a cult, I didn't understand either until I got one (although in my case, I didn't understand why people would want one instead of an actual book, not instead of a phone).

Honestly it's really handy. Super lightweight so I don't get a wrist pain from holding it up at an awkward angle. Battery lasts for months so I can just pick it up and throw it in a bag for a holiday and never think about it. All of my books neatly organised. No notifications interrupting like you would end up with on a phone. Much bigger screen than a phone despite being much lighter, so more text on one page. All of that on top of the reduced eye strain, it's a no-brainer for me.

I have one of the backlit, touchscreen ones these days and the light is useful I keep it on warm light and the dimmest possible for use at night. But I have to admit the older version with the side buttons was better than the touchscreen, I often find myself accidentally skipping pages with the newer one.