It's more than just paying attention: if the conductor can line their finger up with the zebra board for their train, they've guaranteed that the train is completely within the station. Opening the doors where there is no platform is one of the few fireable offenses (others include using a phone on the job). The zebra boards have the train model written on them -- in this case, R160 [1]. While it would be easier for the driver to just stop the train at the very end of the platform, this would be pretty inconvenient for passengers on the end of the platform when a shorter train rolls in.

[1]: https://en.wikipedia.org/wiki/R160_(New_York_City_Subway_car...