Why do time zones exist?
Time-zones are arranged such that at the equator, the sun will be overhead at mid-day. Since the earth is a rotating sphere, one side is always in the sun while the other is in shadow. To keep time relative to the sun's position, the times must be different depending on location.
In the 19th century, as rail travel became common, it became increasingly inconvenient for each community to set its own time purely according to the sun. To accommodate this and other issues, zones were gradually set up so the time was the same within each zone.