"We need to align our instruction more closely with the tasks students face when composing and reading, by including practice in analyzing relationships between words, phrases, clauses, sentences, paragraphs, sections, and the work as a whole." from "How to Teach Grammar, Analytical Thinking, and Writing: A Method That Works" by Lynn Sams (English Journal, January 2003). Embedded in this quote is the notion that the process of writing is laden with grammatical rules, that better we know the structure of a language, the better we will master writing.
I agree, but with two caveats. First, for work-related writing, we need to know our job and the concerns of our readers about our job before all else. Then, of course, we need to know how to write to those readers. Second, I do not take Sams's comment to mean that analyzing syntactic relationships among parts of speech and sentences is the only way to learn writing. Combinations of whole language and in-context instruction as well as whole language and phonics, can also work.
Nevertheless, understanding the grammatical structure of language does help good writers in creating paragraph emphasis, sentence variety, suspenseful sentences, and interesting word choice. You can read more about this article and other ideas about writing instruction at the National Council of Teachers of English website.