The parking brake is called the parking brake for a reason: you should use it when you park your car. Many people think you only need to use your parking brake (also called the emergency brake) when parking on a hill or if your car has a manual transmission. This is incorrect; whether your car is a manual or automatic, the terrain is hilly or flat, you should use your parking brake every time you park.
I will let you google the answer on why you should use it.
And then there's the other side of that coin. The parking brake is called the parking brake for a reason. You should only ever use it, IF
you choose to use it, when you are parked. On flat ground there is no need or reason to use a parking brake on a good working vehicle. On hilly terrain it becomes a matter of decision of the user. Anyone who wants and chooses to use it by all means should. It should be their choice. Not a choice forced on them.
The current setup of mandatory APB for everyone at all times, including those who do not want it, would be like having an inclinometer in the car that prohibits the parking brake from setting on less than a 15 degree incline. Sorry dctokyo, you can't use your parking brake on the 13 degree hill. That's not steep enough. Mazda says so. Too bad if you want to. Not allowed. Now explain why you should be allowed to use it whenever you want in spite of what Mazda thinks and we should be forced to use it against our will because of what Mazda thinks.