why are some first round interviews conducted by managing directors?

i got interviews from two really big investment management firms for their analyst programs for graduating seniors. surprisingly, both of them arranged the first interview to be with a managing director (the one in charge of the program) and later interviews with lower-level supervisors. Why is that? Typically, you interview with the more senior people in later rounds. Just wondering why they arrange interviews this way; isn't this a potential waste of the senior person's time if the candidate is basically shit, and on the other hand if the managing director thinks is the candidate is great, then what is the point of having later interviews?

also, please give some tips for how to interview with the managing director, thanks!