This chapter of the Matsya Purana enumerates various incarnations that Vishnu has taken during the course of the Vaivasvata Manvantara (i.e. the present Manvantara). But along with each incarnation, it also lists an "officiating priest". Here are the priests listed for the different incarnations:
Avatara of Vishnu Officiating Priest
Dwaipayana Vedavyasa Jatukarnya
Buddha Dwaipayana Vedavyasa
Kalki Parasharya Vedavyasa
My question is, why is there an "officiating priest" for each incarnation of Vishnu? What exactly is their role?
I can understand Vasishta's association with Rama, since he was Rama's guru, but for instance why is the sage Markandeya considered the officiating priest of Dattatreya (whom I discuss in this answer), and why is Yama god of death considered the officiating priest of Vamana the dwarf?