Why does DVD region coding exist, you ask? According to what the public is being told, such coding is a tool to protect copyright and film distribution rights (in other words, movie studio profits).
Movies are released in theaters in different parts of the world at different times throughout the year. That Summer blockbuster in the U.S. may end up being the Christmas blockbuster overseas. If that occurs, the DVD version of the movie may be out in the U.S. while it is still showing in theaters overseas.
In order to preserve the financial integrity of the theatrical distribution of a particular film, it is not possible (under normal conditions) to have a friend in the U.S. send a DVD copy of the film to the country where it is in theatrical release and be able to play the DVD on a player there.