The movie studios. Well,
according to wikipedia, anyway. Which surprises me NOT AT ALL. I bought a region-free DVD player years back for this very reason. I've only ever had a chance to use it with region 1 (US, Canada) and region 3 (HK, South Korea), but works like a charm.
One purpose of region coding is controlling release dates. A practice of movie marketing threatened by the advent of digital home video is to release a movie to cinemas, and then for general sale, later in some countries than in others. This is common partly because releasing a movie at the same time worldwide can be prohibitively expensive. Videotapes were inherently regional since formats had to match those of the encoding system used by television stations in that particular region, such as NTSC and PAL, although from early 1990s PAL machines increasingly offered NTSC playback. DVDs are less restricted in this sense, and region coding allows movie studios to better control the global release dates of DVDs.
Another purpose of region coding is to prevent release of movies that could be offensive in such regions for cultural, religious, and political reasons. Region coding helps prevent release of such films in sensitive territories.
Finally, the copyright in a title may be held by different entities in different territories. Region coding enables copyright holders to (attempt to) prevent a DVD from a region from which they do not derive royalties from being played on a DVD player inside their region. Region coding attempts to dissuade importing of DVDs from one region into another.