It’s lets you have 3 shifts per day (3 x 8 = 24) for full coverage. That also fits with the idea that it gave you a balanced life. 8 hours at work, 8 hours asleep, 8 hours for the rest.
It was a convenient number below the old, much worse, standard.
People act like 40 hours is some kind of natural law that can’t be questioned.
It's not happenstance. It's something workers fought and sometimes died to win against an incumbent 6 or 7 day, 10 plus hour a day norm.