Why do some websites have a "Continue Reading" button?

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?