Your in London, UK and schedule a meeting for your team at 9:00 in New York, LA, Tokyo and Bangalore.
Who is awake? Who is asleep? Who's workday ended?
You live in Paris, your mother lives in Hawaii. You want to call here when you get off work. Did you wake her up? Is she at work?
Timezones give you a general idea about what it going on at that part of the world. You lose that information without them.