In universities, what are the advantages of having a second department affiliation? Browsing the internet, I found many professors, researchers and scientists that, in addition to their main affiliation to their main department, have also a second affiliation to another department of their university (for example, Russ Altman at Stanford).

Why so many scientists decide to have a second department affiliation?

What are the main advantages and benefits?
Sometimes, a second affiliation has real administrative meaning, for example if a professor's teaching duties are divided between two departments.  In other cases, a professor's duties are in just one department, and the second affiliation is either just an honorary matter (saying that the second department regards him as respectable in their field also) or possibly (depending on university rules) enables the professor to officially supervise dissertations in the second department.