> how is a dedicated ebook reader supposed to be different from an eInk tablet with a reading app?

A dedicated device can be better for avoiding distraction. When reading on my phone or computer I'll often switch apps to look something up or check something, but when reading on a dedicated e-reader I find the friction of switching devices keeps me conscious of the distraction.

If you can stay focused while reading on a multipurpose device, you probably don't need an ebook device. But I happen to appreciate the option.