In authoritarian, police states such as ours, one key to controlling the populace is to know the location history and presence of those the state is concerned about especially, and everyone on general principles.
Political and social activists, such as myself, are at the top of the list. Many works of fiction relate to such monitoring, typically involving embedding a chip into a person. Even in today’s apathetic society (I guess I’m on a roll today), that might not go over too well if people knew it was happening (as opposed to somehow injecting such a device without our knowledge).
The alternative would be to use a device that almost every person keeps with them constantly. GPS sensors in cell phones not only let us get directions and other information based upon our location, but is also saved in databases the government can easily access, legally or not.
Far beyond that, every text message, email and phone call is also saved for the government to data mine.
The amount of information about ourselves that is vacuumed up is now increasing exponentially, as the government can control an increasing number of electronic devices, such as the cameras in our laptops, and interconnected devices like televisions, personal electronic assistants, like Siri, Cortana, Alexa, Google, etc.