The FAQ about comments has the question but doesn't provide an answer.
So when should comments be used? What is their purpose?
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Comments are used for various things, depending on the site. For main sites, comments are usually used to clarify questions or elaborate on an issue. For meta sites, comments are used for discussion.
There's no hard and fast rule, but I choose between answering or commenting by asking myself, "Am I providing an answer or am I just continuing the question?"
Notice that in the revisions of the answer on that post contain edits by 4 moderators, one of which, made at Jul 5 2012 at 14:59 by Shog9♦. He is the current Community Coordinator for Stack Exchange, Inc. and that makes him about as official as it come with regard to ansewrs regarding the sites features, goals, ideology, etc... That answer is a Community Wiki post - everyone can contribute to it normal users, Moderators or even Stack Exchange staff...
If a you were to ask a "moderators/SE who have designed the system." about this, I believe that they would recommend that post as the canonical answer to the question "How do comments work?"
With regard to their purpose, see the first paragraph -
Comments exist so that users can talk about questions and answers without posting new answers that do not actually answer their parent questions. Comments are often used to ask for clarification on, suggest corrections to and provide meta-information about posts.
The "comment everywhere" privilege page might also shed some light on the subject.
When should I comment?
You should submit a comment if you want to:
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).