What Is The Point Of This Sign?

A number of these signs feature in my local community. They appear and then stay for months or even years. Everyone knows that burglary is a crime so what is the point of letting the burglars know that they may be caught. They know that all ready. We seem to like pointless signs. Like the one on the motorway in front of a large sign covered in black plastic, the sign states: " Sign Not In Use"!