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

A primary interop assembly contains type definitions (as metadata) of types implemented with COM.

Only a single PIA can exist, which needs to be signed with a strong name by the publisher of the COM type library.

One PIA can wrap multiple versions of the same type library. 

A COM type library imported as an assembly can be a PIA only if it has been signed and published by the same publisher.

Therefore, only the publisher of a type library can produce a true PIA, that can be considered as the unit of an official type definition for interoperating with the underlying COM types.