This information can be used to infer things about you, such as your sexual orientation, political positions, personality type, unhealthy behaviors and so on. Then machine learning algorithms can be used to deny you health insurance, credit or employment based on this data.
Or they can sell that data to other companies that will deny you any of these things. So better be careful with out you say, with which books you read, who your friends are and so on.
Or they can make deals with governments to reveal information about you, in exchange for regulatory favors.
Or maybe you're a robot with nothing to hide, but they can still make mistakes or wrong inferences and hurt you in the above ways.
And advertisement can be improved to bypass your cognitive defenses, or catch you at a vulnerable moment (all things that can be detected with enough data) and manipulate you into making decisions against your best interests.
In short, data disclosure can get you killed, poor and/or sick. If that was not the case, why would corporations be so secretive about their own stuff?
By the way, all of the above examples are not hypothetical. They are all based on things that are demonstrably already happening.