I think there is major misunderstanding here.
The Primary-Interop-Assembly (PIA) is a bunch of .NET libraries for including reference to MS Office 2007+. It's for use of Office Automation as what you use for at the time of VB6. The primary advantage of using it instead of directly "Add reference" to the COM DLL is that, it's registered in the Global Assembly Cache (GAC) and it'll automatically load the correct version of Office for you (Many applications in MS Office allow side-by-side installation with earlier versions).