Sometimes in your catalog structure it may be necessary to have categories that are for structural and/or organizational purposes only. You would use these simply to categorize your other categories and they wouldn't typically have products in them.
However, if you want these root categories to show products when customers are navigating your site it is very simple.
Navigate go to Catalog > Categories
Click the Edit icon for the category you want to modify.
Then in the Category Editor, Scroll down to see the Display Settings area and enable the setting
Voila! Now your root categories don't appear empty.