"Those cookies that track you from site to site allow advertisers to know..."
What those tracking cookies also allow advertisers to know is what your interests are. The same advertising agency will place ads on multiple sites. Google, of course, has its tentacles into just about every site on the internet, but others turn up on a very regular basis, as a glance at NoScript's Options list will tell you whenever you visit a site.
Every time you visit a site with an ad by that ad agency, its builds a list tied to a unique ID linked to you. They might not know your name and address, but they know every site you visited that has their stuff embedded in it - in effect, what they have is at least a partial duplicate of your browser history. In fact more than your browser history because if you're browsing in privacy mode your browser isn't storing your history but those ad trackers certainly are. Furthermore, those trackers not only store what pages you visit and when, but how long you spend on each one and some of them even track where your mouse goes and what parts of the page you scroll to. With Google Analytics being nearly universally deployed that means Google have your complete browser history whether you're using Chrome, or in privacy mode, or not.
Ad agencies use this information to "tailor" ads to what they think you're most likely to click on. So if you've been browsing sites that review and/or sell things like RealDolls, sex toys, fetish gadgets and so on, you can expect ads for those things to start appearing even on sites that have nothing to do with them. Depending on your computer-sharing situation, this can lead to some really awkward questions.
Not only that, but insurance companies also pay ad companies for this data. When they correlate it with other identifying information they most certainly can find out who you are. And if you've been browsing health sites looking for ways to stop smoking or asking questions about why you keep waking up at 3AM with a screaming gutsache, you'll suddenly find you're paying rather more for your premiums than most people.
It is this sort of thing that concerns me the most with online ads. I don't want to be tracked and profiled and have a map of my likes and dislikes methodically built up by people I've never met and don't know. I find the very idea of being pried on and a dossier being worked up about me for the purpose of being milked and exploited in this fashion intolerably odious.
And before anyone gives me that worn-out bullshit about "nobody cares about your likes and dislikes and shit," YES THEY FUCKING DO. They don't care on a personal level, no, but they still collect the information and use analytics software to draw conclusions about it. Some of those conclusions will be erroneous, but may potentially lead to problems for me down the track, say when I want to travel or buy insurance or apply for credit, for instance.
That is my primary reason for using adblockers.