Just talking to a cop today who commented on the massive increase of number of number plates being stolen. They are used on cars driving out of petrol stations without paying for fuel. It is not unusual for cars to be found with 6 or more stolen number plates in the boot, false plates are fitted ( usually with velcro or blutack ) the car is fueled up and driven off without paying and the real number plate is then quickly replaced.
One way screws or filling the screw heads with glue putty or silicon rubber is usually enough to deter thieves as they move to easier targets.
This just stops you from having to explain to police why your number plates are on security video driving out without paying from a petrol station.