-preserving the battery on your expensive phone (and thus maybe delaying supporting the horrendous labor practices that go into making a smartphone for another year or so) -less eye strain (could potentially be way better for your eye health long term) -fewer distractions/mode switching
I like my e reader for trips and vacations where I’ll be reading outside a lot due to the lack of screen glare, but otherwise use my phone app. You don’t NEED both as the apps work just as well on phone vs kindle etc, but if you find yourself wanting a bigger screen, no glare, a separate battery for travel, etc then e readers are nice.