Because most of the page is dark, with light only coming from the text, the total light emitted is much less. That makes it less annoying to my wife, when I'm reading before going to sleep.
But indeed it's a feature I cherish.
(I suppose I'm not supposed to feel complimented by the term 'engineer'? I'm not trained/experienced in any engineering discipline, not even software engineering.)
I thought perhaps you had a non-eink ereader and were equating eink screens with ereaders. Apologies.