Why do penger trains have emergency brakes in the cars?

I know, I know, in case of an emergency.....but really, wouldn't just informing the conductor to get the engineer to stop the train be enough? I'm trying to envision what cirstance would necessitate an immediate stop by a penger...all they can see is whats going on on the side of the trains, so the trains movement can't be an issue....what then? a heart attack by one of the pengers.......ok, but the train has to stop immediately ....why? surely continuing on to the next town would be the better thing to do...cpr doesn't require a motionless train....I know these things have been parts of trains since pretty much the beginning...but what is the reasoning behind it?.....why should every penger be empowered to stop the train? whats the worst that can happen where this feature is indispensable. I can see it being important in computerized metro systems if someone got stuck between the doors....but regular rail?