Sometimes, you want to make everyone submit to a subforum on your site and not to a main forum. That leaves the main forum blank except for the subforum links. Perhaps you'd like to use that space to display all the topics from the subforums (and subforums of subforums, etc). That way, viewing the top level forum shows the visitor an unfiltered list of everything in the forum while clicking on a subforum filters to that subforum.

You can do this by going to Admin Panel -> Forums -> Forum Settings and setting "Display Topics From Subforums in Top Level Forums" to "yes". When this is turned on, the top level forums will become essential search-based forums displaying topics from their descendants. Submission to top level forums will be automatically forbidden the same as if Admin Panel -> Topics -> Topic Settings -> "Disallow Submissions to the top _ levels" were set to 1.