I've never been able to understand this. I read from screens nearly all day and I've read books for many hours in a row on normal tablets. I have to wonder if this is nothing but a nocebo that will stop being a "thing" once screen reading is more common, like alleged motion sickness from normal television.
People tend to blink less when looking at a bright, backlit screen than they do when looking at a passively lit screen. The eye is dryer when you blink less, and thus more easily irritated.
"Computer Vision Syndrome" brings up 250,000 hits on Google Scholar. I'm skeptical that it's a nocebo at this point.
Even if reading from a backlit screen doesn't bother you, you have to recognize that reading outside is much better on an e-ink display.
I have a Kindle and I like it because it's small, the battery lasts forever, it's single-purpose, its inexpensive, and is usable in all lighting conditions. Of course you can come up with an equivalent list for your tablet, but the reasons will be different and those differences are enough to justify both products existing.
I strongly prefer reading on an eInk display. I work on a screen all day, but I'd definitely choose "Not reading a book" over "Reading it on a computer display" (a paper copy is still the best reading experience, though -- just less convenient for storage and other such things)
The other pro for the Kindle is that if I put it in flight mode, I need to charge it once a month or so.
