What benefit do you get from using a Primary Interop Assembly (PIA)?

PIAs are important because they provide unique type identity. The PIA distinguishes the official type definitions from counterfeit definitions provided by other interop assemblies. Having a single type identity ensures type compatibility between applications that share the types defined in the PIA.

Because the PIA is signed by its publisher and labeled with the PrimaryInteropAssembly attribute, it can be differentiated from other interop assemblies that define the same types.