Graduate school admission: what is the point of having a waitlist of size 50 while intending to eventually offer a position to only a few applicants?
In my experience, most members of admission committees usually agree on who the star applicants are, or which applicants are simply unqualified.  However, there's a lot of disagreement in the middle, and it comes down to essentially arbitrary decisions among a sizable group of applicants who might or might not be admitted.  This is a strong incentive to have a big wait list:


It can function as a polite form of rejection, to minimize hurt feelings among applicants who genuinely might have been admitted under other circumstances.
It plays the same role for admissions committee members.  If someone is strongly in favor of a particular candidate but there isn't a consensus for admission, adding them to the wait list can be a good compromise.  Insisting on rejecting them outright just leads to pointless arguments.


This role as a sort of "honorable mention" is essentially independent of the chances of admission from the wait list.  However, having a large wait list can have important practical benefits even if there's no chance of admitting more than a few people from the list.  The key insight is that the list isn't necessarily rank ordered, with the first few candidates likely to be admitted but the fiftieth having almost no chance.  Instead, who is admitted may depend on the department's needs, based on which of the previous admissions decisions were accepted.  For example, it may take into account the representation of different subfields in the entering class.  From this perspective, a 50-person wait list may effectively amount to 10 different 5-person wait lists, with no one knowing in advance which of the smaller lists will end up being used.