Why is it dangerous that someone knows everything about you?
It is dangerous because when someone knows everything about you, they gain leverage to control you. In the coming system, no one may buy or sell without the mark, so total knowledge enables economic pressure and manipulation, turning innocent-seeming data collection into a threat that will haunt people.
Les teaches that the desire to know everything about your business seems innocent but is not; it will come back to haunt people that somebody out there knows everything about you. He illustrates from the early 1970s when a private secretary for a huge corporation called a Tulsa hotel to make reservations for her boss. Before she could give his name, the hotel clerk had already punched the computer and knew the kind of room he liked, the rented car he wanted, and where he liked to eat. That was over twenty years ago, and today they know all about you. This connects to the prophecy of verse 16, where the false prophet causes all—small and great, rich and poor, free and bond—to receive a mark in their right hand or forehead, and no one may buy or sell except he has the mark of the beast, or his name, or his number. So the danger is that such comprehensive knowledge becomes a tool for control, especially economic control, restricting buying and selling unless one complies.