As TnE said, fingerprinting. Cookies are a trivial part of it.
Imagine someone wants to track you and target you. For this they need a whole bunch of information about you, and it's a list you would never give.
So they start with one stake in the ground, your browsing fingerprint. In fact in the beginning it is not unique, but it is rare enough for that not to be important.
Everytime you go somewhere with that fingerprint, you are identified and a bunch of data attached to that fingerprint.
What could they find over time? They know where you eat as you left a review, you searched for particular cars on Autotrader, you bought beans from Lidl, you visited particular sites of particular interest or hobbies, you liked something in particular on Facebook, you searched for something on Google - etc. etc. etc.
If you click on a link, then the receiving site knows where you cane from and gets handed a bunch of information (the stuff after a question mark in a link)
Of course, Site A is only a DIY site, and they're not into that, so they sell your data to someone who buys lots of data. They can then build a frightening profile of you, which they can then sell.
Every time you visit the internet you are adding to that profile. And if you use a different browser or computer, in time that will get added to the same profile.
And of course, the bigger and broader the site the more varied the information they gather and the more complete, and thus valuable it is.
Why do you think Facebook, for example, can charge so much for advertising? Because they can target it at a level you'd really rather not know about.
The more data they gather, the more they can charge for their advertising.
Everybody is doing it. Either to use the data themselves or to sell the data to someone who will.
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How Detailed are Internet Cookies / Track.. new
How Detailed are Internet Cookies / Trac.. new
