Private messages, home address, location data. I convinced one of my friends it's like if a stalker follows you around all the time seeing where you're at, snooping over your shoulder to see your messages and what apps you open up, what you post on the internet.
Cops know that if they follow someone around enough that they'll eventually break the law.
Political power can change hands and suddenly you can be part of a targeted group because of what you've posted and said online.