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

You can do this by going to Admin Panel -> Categories -> Category Settings and setting "Display Listings From Subcategories in Top Level Categories" to "yes". When this is turned on, the top level categories will become essential search-based categories displaying listings from their descendants. Submission to top level categories will be automatically forbidden the same as if Admin Panel -> Listings -> Listing Settings -> "Disallow Submissions to the top _ levels" were set to 1.