Pure black doesn't feel like I'm looking at a box light and makes transitions between text and media much more consistent. I end up futzing with the brightness for mixed media (like embedded videos or images) far more when using a gray dark theme, since the true brightness will be much more intense for "media white" than "text white", and the perceived brightness between them will be even higher than that.
Dark themes used to be about practicality and health. Current gray dark themes seem to be born from poor contrast ratio LCD limitations and justification for the aesthetics around the old Holo theme. As someone that has religiously converted their most common sites to dark themes with custom CSS (a few hundred as of now), gray on gray themes feel foggy and I only use light-but-still-dark gray backgrounds on sites that change frequently so I can see if there's a new text element that needs to be styled.
(But, I'm probably in the minority. I also drop font sizes to reasonable levels, delete rounded corners on elements, and remove the miles of whitespace from sites unwilling to accept that a two paragraph "story" isn't worthy of the engagement metrics they get by wasting the reader's time scrolling.)
White on True black UI has way too much contrast and is much more fatiguing than white on dark gray.