Maybe I shouldn't admit this, but it's news to me that you should *always* use a parking brake with an automatic, even on flat ground. I've only ever used it on hills. Never was a thing I was taught, by driving instructors, parents, no one.
Eh, it's more of making it part of a habit than it is "always use it" thing. If you make it second nature then you won't have to remember to use in situations where you actually need it. Hell, turning on my headlights is second nature to me so I pretty much always drive with them on, even in the daylight.
There's also argument that it helps reduce strain on transmission parts responsible for holding the car in park and using the parking brake keeps it working properly. If you rarely use it the cable and mechanical parts of it can become rusted/useless meaning either needing a repair or it won't work when you *really* need it. It also puts two lines of failure between the car and an occurrence that'd cause the car to move. For your car to now move TWO things have to fail, the transmission parking pawl AND parking brake.
It's just good practice to have.