[annnnd]

In the old days this technique was used to get information about number of people who found the article interesting enough to read the next page, all without JavaScript. This of course assumes that readers notice the "next page" link, where is where they seem to have failed (or maybe the expectations of readers have changed so much that the new breed of readers no longer notice such links - quite possible).

[radicalbyte]

It was used for ad impressions. Each page read = new ads = more $$ for the publisher.

[mikepurvis]

For better or worse, it also improves analytics for the publisher— they can measure what proportion of readers made it to page 2, and of those, how long they spent on page 1.

(Yes, some of that could be done with scroll tracking too, "clicked to view page 2" is still a very strong signal of engagement.)