Knowledge of law for citizen and others is required to know how the state/society  wants /directs him to conduct himself and how his interests are viewed in a particular time frame.social frame,political frame or geographical frame. Law defines the relation of an individual and his dignity,status with the society.Law is the manifestation of state's will and vision that governs the society.
Executive wing has to administer it.
A Judge has to examine why the law is there and how the law has to be applied in the given circumstance to achieve maximum compliance of the law so that the aim and object of law can be secured. Law is only a road map and a judge has to travel through it and build the mansion of justice that may inspire society and individual.