Plenty of reasons.

An e-reader is essentially what a book was 20 years ago, except it's an entire library.

Using it doesn't deplete your phone's battery, and a good e-reader can stay powered for a week or more with constant usage. Your phone, given all the background services etc., would die within 8-10 hours.

Then there's the display. Better aspect ratio for reading, better approach for readability (eink), better optimised for reading (cold-warm frontlight), the list goes on.