Telling a story through dialogue is a way of telling it directly from the characters’ perspectives, and it puts the reader right in the center of the story, as though they are on the scene themselves, witnessing the action first-hand.
The first paragraph, however, as well as various other sections of the story, is narrated as though by someone witnessing the scene in which this boy attempts to rob the “large woman.” This choice sets the scene and provides the context we need to understand the exchange that follows.
It is through dialogue that we get an impression that the victim in this scenario is a caring, maternal sort of person.
I got a great mind to wash your face for you. Ain’t you got nobody home to tell you to wash your face?
Through this statement and question, we start to understand that this woman is more concerned about the boy’s wellbeing than about the fact that he has just attempted to attack and rob her. This would have been hard to believe if the narrator had told us this, but as we are told directly by the character, we believe in her concern without a second thought.
Similarly, it would have been hard to believe that at the end of it all, the woman gives Roger the money he needs to buy his suede shoes. In the case of “Thank You, Ma’m” the choice of dialogue makes the implausible plausible.