I know that my partners/customers/colleagues in all countries are available for calls from roughly 9:00 AM to 5:00 PM without ever having to ask them.
That's basically what TimeZones do for you - they provide a common basis to keep everyone in sync around with world, without me having to check, on an office-by-office basis what UTC hours they are in. It's just agreed that 9:00 - 5:00 is a reasonable time to schedule a meeting, and TimeZones tell me when 9:00 - 5:00 is for any particular office.