It's because of such rules the police won't enter your home without a warrant. If they did so, and found evidence of a crime - it would be thrown out. These rules exist to make sure the police have no incentive to enter your home unless they have a warrant (or evidence of a crime in progress)
It protects the population at large from an unrestrained law enforcement system from intruding upon you in a multitude of ways.
It's designed to try and avoid a moral hazard where a rogue police office may choose to suffer the consequences of an illegal search (if indeed a case could be brought) in order to get the evidence he or she needs.