What is an empty sub-assembly and what is it used for?

You can insert an existing or new part into an existing assembly and the same way a new existing or new subassembly. That means you can create a new assembly inside the existing assembly. When you choose that option, you will have to start with an empty sub assembly and build it up either from new or existing parts. All within the existing assembly you're working on - a method of top-down modeling...

more: http://help.solidworks.com/2012/English/SolidWorks/sldworks/Working_with_Sub_Assemblies.htm