I think the idea is that (ideally) these low level details are unpredictable even for a boring C program. I.e., when the C program calls malloc, it doesn't act

I think the idea is that (ideally) these low level details are unpredictable even for a boring C program. I.e., when the C program calls malloc, it doesn't actually need to know what the exact format of the heap is. And if every running system has a slightly different format of heap and stack, that makes it harder to write exploits which will work across all of that variation. Can't smash the stack if you don't know how its formatted.