Onecomment restricts users to posting only one comment per node.
Perhaps you have a petition content type, which users should only be able to sign once? Onecomment allows you to ensure this. There is real power here when combined with fieldable comments - for example, you could have an event type, where the comment consists of a RSVP status field (like on Facebook: attending / not attending / maybe) - you can use onecomment so that users have a single RSVP status attached to a node.