Rabin has a messy office—which is lucky for Verbal. If Verbal hadn't told Kujan his story in such an office, he might not have been able to find enough material to fabricate his stories. Fortunately for him, Rabin's office has a bulletin board packed full of little details that feed right into Verbal's story.
Aside from providing the office, Rabin doesn't play a huge role in the movie. He mainly just hangs out and supports Kujan during the interrogation, occasionally acting as an information tool. For instance, he tells Kujan that,
"This guy [Verbal] is protected—from up on high by the prince of f***ing darkness."
Little does Rabin know that Verbal actually is the prince of darkness, a master criminal routinely compared to the devil.