Ad fraud, or also click fraud, account for a large part of the billion dollar ad industry. Ad fraud malware essentially simulates the user visiting pages with adverts as if they were legitimate views.
All these requests are made in the background and game the advertising system while the victim is none the wiser.
For more details on this threat, check our blog post: https://blog.malwarebytes.org/malvertising-2/2015/01/major-malvertising-campaign-hits-sites-with-combined-total-monthly-traffic-of-1-5bn-visitors/