Here’s a hypothetical store in a place where, say, 9:00 is now 23:00 using global time. The store would have been open 9:00-21:00 Mon and Wed, and 10:00-22:00 on Tuesday. But with global time it would look like this:

Mon 23:00 - Tue 11:00

Wed 0:00 - 12:00

Wed 23:00 - Tue 11:00

Not to mention the general headache of having the day change over in the middle of the day every day. “Meet me tomorrow” when tomorrow starts at lunchtime.

Plus, although you’d easily be able to set up international meetings in terms of getting the time right, you will have no idea whether any given time is during work hours in the other country, or even if people would be sleeping. Instead of having time zones you could look up, we’d have to look up a reference chart for, say, when lunchtime is in a country and extrapolate from there. Or imagine visiting a country and you need a book to figure out the appropriate time for everything throughout the day.

Books that reference time would all be specific to their time “zone”.

It would make so much sense to have a universal time that everyone can refer to for that use case of wanting to schedule things. And, in fact, UTC already exists.