The second entrance to the 191st Street station is located on top of the hill, and the only way to get from there to the train platforms is to take an elevator. There are four elevator cars, and one of them has a permanent human operator whose job is, basically, to press the elevator buttons periodically. It may seem silly to pay someone to do that, but there are a lot of people who feel much safer taking a long elevator ride if there's someone there to keep an eye on things.
The pedestrian tunnel connects to the elevator area outside of fare control, so people who are not riding the subway can still use the elevator as an easy (and free) way to get to and from the top of the hill.
Apparently the attendants (there are four other stations where they're employed, all in this hilly part of Upper Manhattan) used to be allowed to decorate their elevators, but the MTA has since cracked down on that freedom of expression.