Ads whose performance brings a dev box to its knees, ads that cause content to jump around as they load, ads that bloat your screen to the point where you have to scroll to see anything of use on the page you're on, and (this is a matter of taste), an atrocious layout.

Compare, a few pages on, say, http://poewiki.net (the community fork) with http://pathofexile.fandom.com

Compare the loading times. The jank. The layout, that optimizes providing you information, over serving you ads. The usability difference is night and day.