Cookies can be used to record your habits in the browser when you have installed some software that will watch your habits, or when you are browsing websites that have secretly or openly agreed to participate in the scheme, and have installed "watcher" software on their websites.
Specifically, what actually happens is that the network of participators rope a new website into their web, under the guise of "targeted marketing" and such bull sheet.
Then, the new website is told to run the innocent software on their web pages, and the innocent software gathers information that is sold on to spammers, while a tiny contribution is made in the form of pretend targeted advertising.
So, if the cookies did not exist on your computer, this kind of "network" could not work, and that's why there was such a big fuss over cookies.
Here's a worst-case example of using cookies to manipulate the user:
- The "network" adds a startup search website to their web.
- The startup search website is forced to run the "network"'s code.
- The network monitors what the people are searching for, and actually creates malicious websites that contain what is being searched for.
- The user does the search, and gets the results and is now redirected to the malicious website where the fun really begins.
Similar examples exist in the news industry, where the user is redirected to the malicious website on a fake link highlighting a single word or phrase in an article.
i.e. Cookies gather information on your machine which can be used for many purposes. They are not malicious in themselves, but, they can be used to manipulate and deceive, but, mostly, they're cool.