another with the same medium. So, I'd like to know how it makes sense.
Because it gives control over international release schedules. For a variety of reasons studios (and not just Hollywood but studios from any country) don't or can't release a film at the same time in all markets. Region coding gives them control over these schedules so that they have an opportunity to prepare dubs, co-ordinate promotion, re-edit, avoid conflicts with other films, hit local holidays, and of course eventually maximize the audience/$$. Copy protection is a bad idea because it only inconveniences regular users and not the pirates it's supposedly aimed at but conversely region coding only inconveniences a tiny fraction of users who know what they're getting into. If somebody has a DVD that they can't play that's because they deliberately bought such a disc. Anyway, it's easy enough to buy a multi-region player, even in the US. This isn't intended as a defense of region coding--which I don't like--but just an explanation of why it works the way it does.