Man you are dense. People that already know the website by name are not new traffic and aren't who SEO should target.
If someone types in a specific key word related to one of the sites categories and or discussions the results might reveal some relevant information or dialogue that may attract new users to browse the site. That is the point of SEO. That is the point of having categories that impart some idea to a search engine what type of information they contain. This creates relevancy to the user.
Nonspecific categories or irrelevant categories are useless in this regard.
But whatever. I use view active topics and don't pay much attention to the categories. Why they would ever annoy anyone is beyond me. Their only purpose is to classify content in a manner that makes the information easily accessible via search engine.