Previously, you might have only seen “Add Answer” and “Add Information Request” (or others; the exact buttons vary), both of which add a comment and cause the question status to change. But often, for example, all you want to do is clarify an earlier comment, add some detail, or give a progress update. For that, “Just Add a Comment”.
It’s been put at the rightmost position of all the buttons because we think it should be the least used option. Normally it’s appropriate to use one of the other buttons to move the workflow forward.
The button will land in production with the 3.1.10 release next week.