Why does Baldwin make known that the narrator is a high school math teacher?

It is important to the narrative that the reader understand Sonny's brother has taken great care to better himself. Unfortunately, in the act of bettering himself, giving his family a better life, he also removed himself from his roots. The narrator turned away from his heritage. He sought respectability in the white world and forgot his own culture. In disconnecting himself from his past, he's also disconnected himself from Sonny, which has left him unable to empathize with his brother's situation. It isn't until he hears Sonny play that the connection is reestablished... the connection to Sonny, to his past, and to his culture.