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?
Well the idea is that the sun is opposite of your location at 24/12 pm. That way when someone gives you a number, you can have an idea of the lighting situation and such. I do agree that it be better for everyone to be on the same time though. The only issue there is that only one location will have a date/time correspondence with the rotation of the earth. Some places will change the date during the middle of the day. 24 hours will could be any time.