Traditionally the time we have designated in the modern working world as when the sun is over the yardarm, 5 p.m. suddenly matters more than just when you go get a beer and settle in to wait patiently for the next hockey game to start.
There are a lot of things in the NHL CBA that set 5 p.m. New York time as a time of significance. It’s the time when a new day begins in the NHL for things like registering contracts or trades. It’s also when the salary day flips over to the “next day”.