Some article websites (I’m looking at msn.com right now, as an example) show the first page or so of article content and then have a “Continue Reading” button, which you must click to see the rest of the article. This seems so ridiculous, from a UX perspective–I know how to scroll down to continue reading, so why hide the text and make me click a button, then have me scroll? Why has this become a fairly common practice?
What you’re talking about is called lazy loading. It loads text first and CSS and then images after.
Most modern sites now do this along with needing to load it at all until you hit the continue button. That not only reduces your browser load, it also reduces server load as well.
There are many other reasons to have the continue button, but the positives outweigh the negative. It’s not considered a dark pattern and helps the content team improve on their content.