Many people think that one of the important things English courses and English teachers do is "correct" students' writing, showing them where their errors occur. I think, however, that under normal circumstances there's almost nothing you can do that's less useful to anybody. There are people who can learn from having their grammar, style and mechanics fixed, or having their errors pointed out, but almost always they are people who are going to learn anyway, and already understand the fundamental issues. Most people think it's somehow "good for them" to have their errors "fixed," but they go on making them anyway. For example, many people think "alot" is one word; many others think "it's" is the possessive form ("grammar at it's best"). I cannot remember a single case where my correcting the error preventing someone from making it again on the next paper, and I can remember alot of cases where the error cropped up again in the very next paper. It might be that if we sat down and had a conversation about the error you wouldn't make it again -- but given the number of students and the number of errors, that's not a possibility. Many English teachers make serious mechanical mistakes -- writing "principal" when they mean "principle," for example -- and the only way to get it right is to have more than one person check your writing. We all have blind spots. And they're very hard to fix; much harder than a text. On the first draft of my doctoral dissertation, my adviser corrected my spelling of "judgement." Or maybe it was judgment; to this day I can't remember, and I don't know which is appropriate -- one's British and one's American -- unless I stop and check it with my spell checker.
I think grammar and mechanics and style (it's called "copyediting" in the trade) is much like plumbing, in a lot of ways. The most important similarity between them is that they're important, but you really don't have to know them yourself. You can hire a plumber, and you can hire a copy editor. A copy editor, like a plumber, can fix what you've built so it works right, and can tell you how to build so that the building will work, but in order to create a building you don't have to be a professional plumber. You do have to know where plumbing is important. And, of course, you have to use plumbing . . .
(By the way, I'm an excellent copy editor. I don't think that skill
is the most important one I have, but I think it should be paid about what
a plumber gets paid. Unfortunately, it's usually paid a lot less.)