Some comments engage you in a kind of dialogue. Some dialogues trigger emotions (make you laugh for instance).
Can comments be used to share these emotions ?
From the help tour:
Use comments to ask for more information or clarify a question or answer.
Comments are generally to improve the quality of the question or answer, either by directly suggesting a change, asking for clarification or adding additional relevant info. We generally don't like too many "dialog" comments either--back and forth should generally be restricted to clarifying an issue (Fix this. Is this better? Could use this still. Now it's perfect.)
Stuff like "+1 great post" "Very funny answer!" will generally be removed (and should be flagged as noise to help keep the site clean and concise).