It also enables common culture. If someone says they got up at 5am for a flight then it is widely understood that that is pretty early for a typical person, without having to explain as much in words.

If someone says they didn't get home until late, you know they probably mean 9 to some small hour of the morning etc.

Yes! Whenever people say that we should have one timezone I always bring this up. Either you have to look up what time it is in another country or you have to look up what time of day it is. Either way you still have to look it up.

Somewhat, but it does so poorly.

If you're scheduling something with someone non-local to yourself, you need not only know their time zone, but you also need also consider their latitude/longitude and what time of year it is so you account for the potential use of daylight saving time and their current sunrise/sunset times.

For example: if you schedule something at 7PM - is that daytime or nighttime? Well, if it's the first of October, and the person is in Kansas City, the answer is daytime. However, if they're in Phoenix, the answer is nighttime. And here's the kicker: the fact that Kansas City uses daylight saving time is irrelevant, as it would still be mostly light out even if you used standard time due to its higher latitude.

In fairness, that's not what they're arguing for. They're advocating using UTC for yourself, and since no one else does that you'll inevitably get good at conversions to other time zones.

If they happen to work the "average" schedule for their location, and you know what location they will be in at the time of the meeting, and you don't mess up the conversion.

But if you try to schedule a meeting with someone they will immediately know if the time is reasonable for them and let you know.

Plus I don't see how it actually makes a difference, you just look up "what are business hours for $city" rather than "what time is it in $city".