[eli173]

You don't need to be tracked for this to happen though. If ad networks placed video game-related ads on video game-related websites and tampon-related ads on tampon-related websites, then users of both sites would get ad content relevant to their interests.

[cheald]

Consider contexts like news websites, though, with broadly-applicable audiences. Specialty or niche sites do just fine with site-derived advertising targets. Other things do much less well.

There's also the fact that targeted ads result in better CPMs for publishers (since they'll result in better returns-per-view for advertisers), which means that they don't have to deploy as many of them to meet revenue expectations.