Guessing, but probably to get more accurate engagement stats. It would be better to assume a user has read most or all of the article if they clicked that button. Maybe also a way of determining if the introductory paragraphs were written in an engaging way, because they determine whether I will continue reading or not.
Seems like monitoring the scroll position would have the same (or better) effect.
It has a paywall so when you hit the limit of 10(?) free views that no longer works.
OK that makes the most sense - I've just never seen the paywall.
It's a common "recirc" technique (recirculation). All that crap after the button/article? If you skim the first paragraph and are not interested, they want you to go there rather than press "back".