The font size is adjustable.

Even with glasses, I often find that the print size in a book is too small to read from a comfortable distance. Using an eReader, the font size can be adjusted to whatever suits me best.
The screen brightness is adjustable.

Thefore, I can read at night without an external light source. In addition, my eReader, the Kobo Clara HD, contains a setting to adjust the natural light which can reduce the amount of blue light exposure at night.
Conveniently look up the definition of a word.

Highlight a word to easily look up its meaning rather than pulling out a separate device.
Find previous references to a character, place, etc.

From time to time, and especially when returning to a book after a hiatus, there may be a reference to a character or a place that I don’t remember. Highlighting the word and selecting “Search in book” pulls up all other references to the same word within the book, allowing me to jump to previous references in order to regain context.
Direct access to free eBooks via the public library system.

I’m able to connect my eReader to Overdrive where I can access the electronic collection of a couple public libraries which I belong to, including the Brooklyn Library (available to out of state residents for $50 / month). Then, I can checkout a book and start reading in minutes, without even leaving my home.