It's nothing to do with rights (for example, Australia, New Zealand, and all of South and Central America are in the same region). Studios like to release movies on different dates in different parts of the world, for a variety of reasons (some even almost make sense, such as extra time for translation, but the delays even happen when none is required). The region codes are there to protect ticket sales at the cinema by making it harder for people in the later release countries to buy a disc in advance.