I also read some things in the languages I'm trying to learn, and this feature is essential for translating words I don't know yet.
Plus, the backlight (adjusted for warmth so as not to keep me up) lets me read in bed with the light off and I don't have to get up to turn it off when I'm done.
I was very pro only real books for a while, and still am a fan of a real book, but the quality of life features of e-readers eventually sold me. Especially because night is one of the few times I can read.