There is plenty to learn about you. They can experiment on you and see how you react. In fact, they do. Those seemingly random videos they throw into your feeds is just them using you as a lab rat. They throw it and see your reaction. Will you click it? Will you search something related to it? If they recommend you a video about how your privacy isn’t worth anything, will you write a comment restating it a few weeks or months later without remembering that video? There is lot to learn about you.