There's some overlap, but ads aren't the only thing using this. Google Fonts is the classic example of something that does add value (nice fonts) to a webpage, and as such isn't blocked by most adblockers (including uBO) by default, but is still able to use this for tracking because the same domain serves the font files on every website using them.
Very insightful response. The tendency is to look at the cookie problem as an advertising problem. Whereas, there are other cross-origin use cases that are not adverts but pose the same tracking threat. Anything on a CDN (like common javascript libraries) share this trait.
