My mom was a high school English teacher and I had correct grammar in both speaking and writing drilled into me since I was able to speak and write. This has been a huge asset to me as an author. I often, but not always, know when grammar sounds/looks/is correct or incorrect, but I don't always know why it is.
I have two writer friends, amazing women, who help edit and critique my work. Both of them know everything abut grammar. When they edit and critique they don't just write: "This needs to be worded this way," or "This sentence has to be structured like that." They explain why and cite the grammar rules that apply. One of them includes "for further reference" the exact section of the Chicago Manual of Style where the rule can be found!
Sometimes I want to be like them. Other times I think it is much more practical and useful just to know them and be forever grateful for their knowledge and willingness to share it and educate fellow writers. I am blessed to have such incredibly talented people in my group of writing peers.