Even if the whole text is loaded with the initial page, you'll see a request to somewhere to record that you clicked. Your engagement has been measured. This can be helpful for the site directly (which articles do people actually care about after the first paragraph) and that people are engaged enough to click for more is something they can “sell” to advertisers. A better designed site will have the “read more” button be an actual link so if you have JS disabled (or it fails to load) instead of the content reveal simply failing it falls back to a full-page round-trip so you are counted that way.

This could be done with picking up on scroll events or visibility tests on lower parts of the article, instead of asking the user to click, but those methods are less accurate for a number of reasons (people with big screens, people with JS disabled or JS failed to load, …)