There are about 50 different types of work which a police officer can be assigned to, and almost all of them rely or depend upon the authority of a uniform.  One of the most notable exceptions to uniformed duty is undercover work, arguably the most problematic area of law enforcement.  Undercover work is one of the most unique investigative techniques available to law enforcement.The theory behind not wearing a uniform is that it removes any impediments to acquiring information.  It's part of the same intelligence-gathering function as surveillance, eavesdropping, use of informants, and espionage.  It typically involves an assumed identity for a defined and considerable amount of time.  Undercover work requires secrecy.  It allows the police officer to circulate in areas where the police are not ordinarily welcome.