It's why Facebook and google are worth so much, because of their data. They have data on anyone who has ever visited their sites because of pixels. This piece of code tracks all of your movement throughout the internet with or without your concern. Using this data they sell it to advertisers. It's why merchants and advertisers love them because they can target users based on interest and demographics. Also create audiences called lookalikes and target people similar to their customers and retarget people who visited their websites because they have pixels installed on their sites as well. It's all about data in this day and age