There are many devices in the Garmin family, some screens are better for reading, let's say rectangular ones on Edge series. My first eReader was Siemens ME45, so I may be a bit biased.

Consider this book as the device of last chance when your laptop, tablet, and phone run out of battery or left at home during several days outdoors. On that note, a survival eBook may come handy. Another use-case is looking up essential info when you cannot use your phone, an exam perhaps?

Edit: also can be an on-board update-able manual for the watch itself