Most of the time when I edit a question it's to put a quotation into block quotes, or to clear up some other problem with presentation.
Sometimes, I will edit to correct an obvious typo or glaring issue with grammar.
However, I never edit for style, to rewrite, or to fix systemic grammar issues. I don't believe that more than minor content corrections should be made. To do more than that is to take away the voice of the author without their express permission.
Some editors are employed to make stylistic changes. But when they are, either the authors have spelled out general guidelines for what changes they want to see, or the editors are working under the guidance of a publishing house and its particular house style-guide.
For context, I should add that I am a professional editor. A common mistake of editors, especially those who do not have years of experience, is to assume that everything should be "fixed" according to a particular style (most often their own). In reality, there are many different approaches and styles.
As time goes by, I learn more about how things can be written. But I also become more accepting of the number of widely differing opinions about how things should be written. In editing, everything being equal, it's the authors who are the owners of content in the final analysis. Editors should be helping them, not writing for them.
For the most part, editors should only correct glaring issues—unless they have been specifically requested by an author to do more than that. But what's glaring can differ based on different scopes of work.
A key part of editing, which isn't possible on this site, is the use of comments side-by-side with the running text. Even if you don't change any text, you can still comment on it. ("I suggest this phrasing," or "Are you sure you didn't mean to say this instead?")
When I see a post that doesn't appear correct to me, I seldom actually edit it. Instead, I leave a comment under the post asking if it's what the author had meant to say.