Several stages of the interview

Greetings to all. I learned that now it is “fashionable” to hold 2 interviews while applying for a job: one with the head of the department where all technical issues are resolved, my competence is checked, etc .; the second with the director. Question: what is it, why, what questions will be raised? I guess that first of all it is s / n, but what else?
Thanks a lot in advance.