That's a misconception. ATAR's aren't actually set (few exceptions but lets not focus on those). They dont have a committee meet, throw darts at a board and set the ATAR on what it lands on. It really purely is a measure of supply and demand. The eligible applicant with the #1 highest score gets place #1, applicant with #2 highest score gets place #2 and so on until they fill how many students they are willing to take and think they can hold.
Obviously, it's thats very simplified and they adjust for things like SEAS, middle band and especially any courses with UC/folios ATAR's aren't all of the story. For your regular courses like BA or BSci thats how it basically works. People can be below the clearly in and still get in but it is a generally useful guide for what you should aim for. I'm not sure if it's actually the median like dentistry suggested, that would suggest a massive amount of people getting in under the clearly in which doesn't really make sense.
The only thing that makes last years ATAR useful at all is that demand TENDS to stay constant from year to year. Sometimes there are unexpected jumps or drops, i remember in my year the clearly in for UoM went up 5 whole points which is a pretty massive jump. They didn't really decide that ahead of time, it's just how it panned out.
TL;DR Useful guide based on who got in last year. It will be different this year. For most courses in most years differences between the years aren't big so thats why its a useful guide. Sometimes the jumps can be rather large though.