It's a deprecated design practice to make font rendering looks smoother in a world where retina screens were not ubiquitous. Back in the old days (so like 3 years ago) before the majority of screens were 'retina' (aka you can't see the pixels at normal viewing distances) fonts always rendered with jagged edges. Turning down the contrast between a font and its background is effective in reducing this effect. It reduces the contrast, it's a very straightforward mechanic. Obviously, this is moot now with razor font rendering on retina screens. And if you do it too much, you risk accessibility problems.
The nonsense about black on a white background being too harsh because of eye strain is exactly that: nonsense. If you wanted to reduce eye strain, you'd dial down the brightness of the actual background, often seen in "nightmode" options offered in various UI. (Come to think of it, that would be hilarious if you hit "nightmode" and it just lightened the text, making the overall screen even brighter.)