It means that you can keep your eyeline focussed on the middle of the viewport (or the top, or wherever's natural for you) as you scroll new content into view. On a regular page, once your scrollbar reaches the bottom you have to start reading down the viewport because there's no more space to scroll. I don't know how much practical difference it would actually make to readability - would be interesting to measure - but I thought it was neat

When you scroll to the bottom while reading text you always only need to look in the middle of the screen when it's padded. Otherwise at the end of the page you would need to look at the bottom of the screen

When many people read an article on the web, they keep their eyes in one small vertical area on the page and scroll to shift the text (rather than reading the whole visible area and then scrolling to the next "page"). However, when you reach the end of the scroll area, you're suddenly forced to move your eyeline down the page. Adding a bunch of space adds means you can keep scrolling the text to be underneath your eye-area. (sorry for the crufty explanation lol, it's kinda hard to visualize)