Profit

It stops people from importing DVDs from regions where it's cheaper.

Because not all currencies and economies are created equal.

Someone in Thailand can live pretty dang well on an income of USD$10,000/year. The median income is about 5K/year (but with much of the nation as subsistence farmers.) Anyway. If you make 5-10Kbucks a year, a $20 DVD is a lot more expensive, relatively speaking.

So companies sold DVDs for much lower prices (at least, in USD) in markets like that. This way, the people in "poorer" nations can still afford to buy the latest Hollywood bullshot and our cultural hegemony is maintained.

But when you have region-specific pricing, that creates opportunities for arbitrage, that is, some US dude could fly to Thailand, fill his suitcases with DVDs and then resell them in the US at a profit.

And that was a Problem. Not because of individual profiteers, per se, but because if the opportunity is big enough, someone will make a business out of it. Instead of Blockbusters on every street corner, you'd have the local Thai import store selling the latest Marvel flicks for half the cost of a US print.

So - region-locked discs. Which were immediately cracked and sued over. And since the internet had really arrived by then, we all took advantage of these amazing new DVD-copying skillz to share movie files for FREEEE. And thus, piracy turned out to be a much bigger issue than region-specific pricing.

There are still regional pricing issues. Valve just axed region-specific pricing in Argentina, for instance, which immediately locks most of the country out of affording games.

I don't have a tidy ending to this. It's complicated and unclear (at least, to me) whether the public interest is better served by regional pricing or not.

But that's the long and stupid story of why OP can't play their legally acquired DVDs tonight.