I can tell you my reasoning. While developing the dark mode theme for my mobile app, I experimented with pure black background first. It looked off. I couldn

I can tell you my reasoning. While developing the dark mode theme for my mobile app, I experimented with pure black background first. It looked off. I couldn't figure out why it looked off (design wise, but also seemed weirdly hard to read) until I played with a bunch of other mobile apps with dark mode (which I used already and liked) and noticed their backgrounds are not black. Then I just tried a couple of shades of "almost black" and the improvement was huge - easier on the eyes and more pleasurable to read. I don't have a scientific explanation why this is the case, just I wanted to kinda directly answer the question with my anecdotal experience of why I do it.