In New York City elevators are legally required to have mirrors.
Per Multiple Dwelling Law § 51-b; NYC Admin. Code § 27-2042, The main purpose of the law is so people may see, prior to entering, if anyone is already in the elevator.
The Law: Multiple Dwelling Law § 51-b; NYC Admin. Code § 27-2042
§ 51-b. Mirrors in connection with self-service elevators. In all multiple dwellings in which there are one or more self-service passenger elevators, there shall, pursuant to such regulations as the department shall prescribe, be affixed and maintained in each such elevator a mirror which will enable persons prior to entering into such elevator to view the inside thereof to determine whether any person is in such elevator.