Advertising on websites
Advertisers already know a fair bit about a site reader by knowing about the site not the reader. If I'm reading TheReg or Slashdot, I'm probably interested in Tech, so advertise Tech related products to me. If I'm reading a Motorbike forum, show me a picture of that latest Ducati.
You don't need to track me individually from the Tech related site to the Motorbike related site just to show me an Intel ad while I'm asking a forum about bleeding brake lines.
I've never had a big problem with ads, except maybe Popups etc which get in the way of the article I'm reading, but I have a big problem with companies I have no relationship with logging every webpage I visit and that desire to avoid tracking is what forces me to run with RequestPolicy etc.
The Online Advertising industry seem more interested in tracking than advertising.