For top programs it is a point of pride to not need to go too far down the applicant ROL to fill the class. It is a surrogate measure of the desirability of the program; if everyone who interviews ranks a program number one on their list, then they would fill the program 1:1. In reality, this doesn't happen, but there are clear advantages to being able to gauge the quality of your class before match day.
Just as applicants rank programs that are not their favorite, programs do the same thing to ensure that they fill even in a bad year. You don't really want your class to be filled by a large amount of these "safety applicants". This is why the NRMP is so careful about post-interview contact, because unscrupulous programs could try to game the system based on the applicants' ROL to fill more quickly with higher quality applicants. This is also why top applicants get notes from PDs telling them how much they enjoyed meeting them, to try to influence their ranking on the applicant's ROL in order to help them rank to match.