When editing my question or answer, I want a blank "comment box" under the "edit summary" box.
The edit summary field is documented by answer https://meta.stackexchange.com/a/84708/242736 to explain "what is edited" and not "why edit was done".
I want to use that additional comment box to explain "why edit was done". Other people also wanted to write about "why edit was done": Why aren't edit summaries shown? (I was requested not to write this feature-request as an answer to that post.)
It would be more convient to fill that comment while editing than afterwards. And without that comment box, readers have the time to be confused and post such comments between my edit and my comment.
For example, in some of my answers to a Tex.SE question, I have taken in account suggestions posted by others in the comments; I then added a comment to thank them, and also to let the reader understand that these comments have expired; it would be better if I could expire myself the comments as discussed in this question.