They can collect all kinds of information on that person, including their interests, what they watch, what they buy. Why do you think all these companies are collecting data on users?
I remember watching an interview with a guy who said that his cell phone knew he was gay before he did. When he was a teenager, the algorhythm was suggesting gay websites for him before he even knew he was gay.
Imagine how that would be used is Russia, or Muslim countries where being gay is illegal.
My first e-commerce website in 2001, I was shocked at the things I could find out about people visiting my website. I could literally stalk anyone and follow their mouse clicks through the site.