A university faculty may have one person with that specialty, and others have different ones, so you interact continually with people from a variety of specialties. It's like NCAR in miniature. NCAR benefits as well. UCAR encourages scientific staff to seek affiliate professorships. Scientists at any level are eligible and costs are shared by UCAR and the university.
Affiliations are usually for a term of three years but can be renewed indefinitely.
At least once in the three years affiliate professors visit their universities for an academic term or longer.