it is preferance.
two different methods of writing to dvd had been made, and the industry favoured one but manufacturers found the seconed was cheaper and easier to produce, so it became a conflict of interests manufacturer versus the industry sort of thing,
the problem was that 1000's of dvd films and players had allready been produced in the main format so they couldn't just scrap it and start over, so the main format stayed for high street film and players + and the pc market found that the 2nd(-RW) was perfect for data storage and cheaper than it's predecessor although it defeated the object of dvd copying. as you couldn't create a dvd of your family from camcorder and put it on dvd and watch it on your dvd player, so dvd -/+ recording unit's where created for the home pc.
so as i stated at the beginning, it simply is the buyers preference.