Not hard to see why. Being an author helps your cv. Allowing you to be an author for tangential or minimal contribs can help keep good relations, especially if there are future options and financial things depending on having good relations. Putting a name on a paper costs nothing and nobody checks how big the contribution was. It's slightly dilutes the subjective authorship fraction of those who did the work, but sometimes the additional person also brings in a nice prestigious affiliation that even has a positive impact on how seriously the paper is taken... It's a game.