All they do is complicate things. In today's society where communication is easier across long distances, we should have a set Earth time. In each respective area, people will learn what time the sun rises and what time it sets. Communication and correlation over long distances would be simpler, and keeping track of world events would also be easier. Agree or no?
The only thing I hate is Daylight Savings Time. Technically, we do have an "Earth Time". It's Greenwich Mean Time. As for why we have time zones, it was because of trains. Time zones made it a lot easier to schedule arrival and departure times for trains. Instead of each station keeping its own time, a whole region would have a designated time. This system naturally was adopted by ships and aircraft over the years. Using a single time and ballparking whether it's morning or night on some other part of the world would make international shipping and transit a huge pain in the ass compared to adding or subtracting a few hours to your time. Right now, I know that if I ship something from New York at 10:00AM, it will arrive in Amsterdam at X:XXAM provided there are no delays.