Third-party cookies are mostly used for web analytic purposes. This can happen if your web browser loads an advertisement or a so-called targeting pixel that is not hosted on the server of the visited website. Your web browser generates an additional cookie, the third-party cookie, because it is not assigned to the server of the website, but to that of the advertiser. Nevertheless, this third party cookie reads all the information that the first-party cookie notes anyway - and sometimes even more.
Because web analysts are primarily interested in user behavior, the third-party cookie usually documents the page history on a website. However, this cookie often gains really valuable data only when it “recognizes” you on another website. Since your web browser communicates again with the same ad server, it can trace your path on the internet, and not only that: your behavior on the web reveals a lot about your interests and your consumer behavior. This creates a user profile that enables targeted and personalized advertising.