We often use Shadow Menus (or often called hidden Menus) in our Joomla Templates and also here at our own Site. The reasons are simple, it's better for a perfect SEO.
If you are using no Shadow Menus Joomla often create URLs like this:
This is not well for SEO and also it makes it complicated to Link manually to this Article. Cause of this we are using Shadow Menu. The menu is “hidden” or in "shadow" because we don’t need to display the menu on the site. We are just using the menu for control purposes. It has also the advantage that you can assign Modules to this Article. So here's a short summary cause we use Shadow Menus:
How to create the Hidden or the Shadow Menu
You can also check this Blog Article at Joomla.org.