This functionality is more intended toward requests/support/questions forums, where users can ask leaders questions. Once they've received a satisfactory answer, they can flag the post as having been Answered.
For admins, the functionality can be set for individual forums on the Admin Menu -> Features -> Forums -> Forums area of the site. When you create a new forum, or edit an existing one, one of the options is to use the Post Is Answered functionality.
If this is flagged, then there will be an 'Answered' link underneath the user information of every user's response to each thread in that forum.
You can click that link, which will flag that post as the Answer to that question in the original post. That way, if there is a lot of discussion on a given thread and one definitive answer, it will add a link in the first post to the actual answer, saving users time and patience trying to track it down. :)