Advertisers have different ad codes for regular browsers, mobile browsers, and mobile apps. They want you to figure out when to serve each. Give the ad codes to us instead, and we do the work for you.
When your site is so popular it takes us significant resources to serve, then we'll ask for compensation. But we won't make any money from your site until you do.
We can insert ad codes into a page asynchronously so that ads appear on pages loaded by AJAX or awesome frameworks like jQuery Mobile. Most ad codes simply can't do that.