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...

Having the driver stop at the end of the platform wouldn't really be a substitute. What counts is that the conductor in the middle of the train physically observes that the train's registration is correct before they open the doors. (Drivers never open the doors.)

Yes, the article we are discussing here spends​ two paragraphs discussing exactly this.

I know this is just a bunch of kids taking the piss, but little things like this make a boring day working on the train a bit happier. Thank your local transit workers if you have them.