<p>This concept is new to me, and a colleague suggested it. Sadly, I had no idea what he was talking about. Can someone enlighten me?</p>

You can find plenty of information about it
here
.
In a nutshell, a PIA is a signed interop assembly that provides the "official" definition of types in a COM library from the publisher of the COM library.
As to the benefits, the posted article sums it up pretty good:
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.