So they can track and analyse the users who visit the site, what content they consume and what products they buy. This information is sold both to the website owner (when they buy ads on meta's platforms) but also packaged up and sold on to companies that make a living from interpretating that data and selling it (down to the detail level of households and/or individuals). Use Firefox+ublock origin, install a DNS adblocker like pi-hole on your network and last but not least: if you have to use their crap, do it in an isolated environment (virtual machine for example).