"I'm in Berlin, my working hours are 7:00 UTC to 15:00 UTC, and I'll be reachable until 20:00 UTC".
Arguably, without timezones, it's much harder to figure out when its daylight in a particular region, so when someone should be reachable.
"Well it's 06:40 UTC, but what time does the sun rise in New York? Has the sun set in Bangkok?". Meanwhile having googled that it's 02:40 AM in NY means I can be assume a normal-working person would be asleep at this time.
Perhaps that's cultural. Most people in my social circles care quite a bit about sunlight. At home, the evening routines that we do with our children are based on a combination of season and sunrise/sunset. I find it somewhat surprising in fact that this is not at least partly the case for others, excepting of course people that work 3-11 or 11-7 or what have you.