Licensing. The DVD publisher bought a license from the rights holder that allowed them to sell a movie on DVD.
That license was limited to sales in a specific market, so they put in a region codec to prevent sales in other markets.
That way the same movie could be sold for a lower price in markets where people wouldn’t be willing or able to pay as much.
Without the codec, you could just buy a DVD in a cheap market and re-sell it in a more expensive one, making a profit off the movie without the rights holder getting a cut.