can't find an article right now for you, but mostly it was fixing bugs, as far as I can remember it was setting the height and probably also avoiding margin collapsing issues but I could be wrong
It is at least for ensuring the page is 100% height minimum. So if you have a page where the contents like header, body, and footer don't take up 100% of the user's viewport, the page wont show white at the bottom where the page doesn't expand to