Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates (for example, the sun being at its highest point every day around noon), different places on the Earth need to have different clock times. Time zones have been used in modern times so similarly situated cities can keep exactly the same time, for simplicity and ease of communication.

On land, it is more convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries.
Read more from source