From The Onion:
It’s a good question. And the answer is pretty simple. It was an attempt to get you to click on CNN.com so that we could drive up our web traffic, which in turn would allow us to increase our advertising revenue.
It's called link-bait. When you visit these websites, they get to deliver ads for you to see - these are called impressions. When you click on the links, that's a conversion. Websites get to sell ad impressions to advertisers for a little bit. They get to charge a whole lot more for conversions. When you visit these websites, or even Facebook or Twitter, you're the product. The advertisers - how they make their revenue - are the customers. They're selling access to their product.
When possible, spend the money on a subscription site. Avoid ads. I'd rather pay for an app and support a developer, or pay a monthly subscription to a website and support an author, than to deal with advertisements everywhere I look.