The reason we have time zones is because the earth is spinning like a top. The reason it gets dark at night is because at that point, you are facing away from the sun. but in the morning, the earth has turned back so you are facing the sun.
It was be very weird if the whole world used the same time. If you wanted to say, "I got up at 7am," no one would know if that was when the sun was out or not unless we had time zones.
The earth takes about 24 hours to make one complete turn on its axis. Since there are 360 degrees in one turn, this means that the earth rotates through 15 degrees every hour. This is why most time zones are about 15 degrees in longitude across. (This is not exactly true...some time zones have pretty strange shapes, and some are only half an hour wide, but it gives you the idea).
In any case, this is useful since it makes people everywhere agree more or less when morning, noon and night happens. If the sun rises at 6am in New York, for example, it is 5am in Chicago where the sun has not yet risen. An hour later (6am in Chicago, 7am in New York) the sun rises in Chicago. People is both cities will agree that the sun rises around 6am.
(republished on 07/19/06)