Moderators have the same reasons for editing a post that other users have. The block that users without the privilege of editing any post see reports the following points:
- fix grammatical or spelling errors
- clarify meaning without changing it
- correct minor mistakes
- add related resources or links
If the edit is done following those points, the edit is correct, and it can be done whenever the question has already an accepted answer or not, as the purpose of an answer is not just helping the user who asked the question, but any other user who is interested in an answer for the same question.
Clearly, an edit that just changes "Quoted sentence with comma, and period". in "Quoted sentence with comma, and period." is merely changing the sentence from the style used in UK to the style used in USA, and it doesn't improve the question in any way. Differently, an edit that permits the users to understand a question without to follow a link to an external site, which could be down for any reason (including the fact the site has been closed), is an improvement.
As for the moderators editing questions, there are two actual moderators who has the Strunk & White badge, which means they have edited 80 posts (including the ones they edited before to become moderators), and which includes answers. There are other 3 users with the same badge; that means the edits done by the moderators are a minimal part.
The edits considered for getting that badge don't include those edits that just change the tags used for a question, but those are just edits done to better classify a question, and help other users finding it, or help other users in finding duplicates; they are not edits that change the meaning of the question (which should be what other edits do too).