You can handle yourself amongst the criminal population. You know how they dress and act because of your experience. This isn't really a skill you learn at school. Your familiarity with city streets makes it easier to track down illegal goods and to find information otherwise unavailable.
You know common tricks and tools of the trade, enhanced by other Espionage skills. You're adept at bribing the right people and are familiar with the syndicates operating in any well-traveled area.