They do that because every time you click next the page is re-loaded to make it look like there is a lot of traffic on the site thus the advertisements get more views. This increases their ad revenue fraudulently; the thing is that it is against the rules of most advertisement companies for websites to do this.
Usually any time I see a website like this I leave. I also use Adblock Plus for this reason as well.
Red 1988 GT under restoration!
Let's Go Mets!
( ͡° ͜ʖ ͡°)