If you press page down to scroll, at the end of the page you’ll typically scroll by a smaller amount than prior to that, which changes the level at which your eyeline should be after hitting page down to continue reading uninterrupted. In Emacs and Vim hitting page down works nicely, because they just scroll “past end”. Other programs, like web browsers, often stop at the end, causing this issue and making the reader look for the point where they stopped reading for maybe even 30-45 seconds. This seems to be a workaround for that.

The page will continue to scroll until the last line can fit to the top of the page. Nobody likes it when you can't scroll past the end, and have to then start moving your eyes/head to look at the bottom of the screen. Same with IDEs.

It means that you don't have to move your eyes horizontally very much. Some people (including me) like to just scroll.