It\'s a sub-title, so you can effectively give a general summary of your post. It\'s true it has to be an extremely brief summary, if your post is more than \"YEAH!\"
I think it might be because entering a comment is managed a lot like creating a new topic, just, inside a topic. Can\'t blame whoever took this approach, it works and can be given a purpose.
Say for exemple, your reply/comment wants to adress a particular point of whatever it\'s replying to. You could use that as that title line, such as \"About your ____ idea.\" etc
If it really bothers you that much, \".\"