For what purpose does tan juxtapose "i am not a scholar of english or literature" with "i am a writer"

Because she wants to point out that she is not interested in grammatical, normative aspects of language. As a writer, she is primarily interested in functional aspects of "all the Englishes" she speaks. She wants to say that a writer will not prescribe what is right and wrong, what should and shouldn't be used. A writer will make the language work. She doesn't analyze the language, as scholars do; she uses, adores and admires it, as writers do.