It might attract customers who still have DVD players only and haven't made the jump to BD. If they are thinking about buying a BD player, they can buy a movie now and still enjoy the DVD copy, futureproofing their collection.
It is also handy for people who plan on car viewing, as most in-car players are DVD and not BD. This is equally true for people with laptops, as laptop BD readers are easily $100 extra and not easily retrofitted.
It's also a way to make the packs more expensive.