Ming Cher's explanation of how gangs function in Singapore has several obvious functions, one of which is to place the author himself and his narrator in a place of authority. Reading the following passage, we assume that the person knows a lot about the underside of Singapore in the 1950s, and we may draw the conclusion that the author has first-hand knowledge of his subject.
Open gang clashes among secret societies happen every day. Rebels within gangs constantly sprout out to form new gangs. They use numbers to identify their groups, number is strength, is the meaning of their movement. It tune with the time, and was trendy, has a macho appeal for young recruits who like to dress up like teddy boys with skintight pants and get into the sounds of rock and roll or cha-cha of Western music. Many copy Elvis Presley who is known to them as King Cat. The Black and White youth gang wear shirt with collar turned up high, tattoos were changed from dragons and godlike warriors to cowboy girls with big tits, English letters, or animals like black panther and snakes. Some adjust their collar to reveal their gang, some stroke their nose with two fingers to say, "I am from Two-Four," or make a zero sign when light a match, for Zero-Eight gang. One of Chai's half brothers belong to the Two-Four gang, the other belong to the Three-Six-Nine. 
Assuming that Ming Cher was himself a gang member, or had some other first-hand knowledge of gang culture, might serve to garuantee the realism, the authenticity, of the text. But we must beware. Think how many readers of Waterland mistakenly assume that Graham Swift grew up in the fens and that he wrote the book when advanced in years -- neither in any way correct! How important is a conviction of the narrator's accuracy to your pleasure in reading Spider Boys?
This passage also serves to locate the reader in a particular historical context, one in which South East Asians had come under the influence of Elvis Presley and American and British pop culture. What other effects do such details have?