If everyone follows the same clock, like, now is 12:30pm, across the entire world, then while everyone could also work 9-5, we know that most people like to be awake during daylight and asleep at night, so the local working hours might be 2-10.

We can get an idea of the issues in China, which has a single national timezone instead of 4 or 5 geographical time zones. From https://en.wikipedia.org/wiki/Time_in_China

> Beijing Time users in Xinjiang usually schedule their daily activities two hours later than those who live in eastern China. As such, stores and offices in Xinjiang are commonly open from 10:00 to 19:00 Beijing Time, which equals 08:00 to 17:00 in Ürümqi Time.[17] This is known as the work/rest time in Xinjiang.

Thus, "9-5" would get a name like "work time", or the existing name of "day shift" would have increased popularity.

Because we set it up so that our shared infrastructure goes under load at the same time instead of being better distributed. With a universal clock, there may be less pressure to start at shared time. Staggered starts.