What is probably unique to online advertising is widespread fraud by sites. If you run an advert on TV, radio, or in the paper, then their circulation will have been audited by third parties to ensure that they're actually reaching the audience they claim to be. There's no real equivalent to that online that doesn't involve tracking cookies. Without the real-time feedback, you have no idea if the ad you just bought is being shown to real readers, robots, or some hidden iframe on another site entirely.