It works like this:
When you create a new card concept, the thread for discussion about it is not created. Only after you push the "Start discussion" button an empty thread with the same name is created.
Such thread keeps the matching name even if you rename the concept and there are shorcut buttons "go to concept"/"go to discussion" to connect concepts and forum sections. When the concept is deleted, it's discussion thread remains.
Why are so many empty threads here? Because someone started a discussion (created a thread) and then didn't write any posts. Those users are the authors of the empty threads. My guess is that they were wondering, what does the "start discussion" button, thus creating an empty thread out of curiosity.