generally, its mainly to show relative differences between students in your subject. Because marks cannot be compared between different schools due to varying difficulties in exams/assessments.
If you got 40% and came first it doesn't matter. When you are ranked 1st you are totally immune to changes that can occur from the rest of your class.
So if you got 40% and ranked first, it may show your exam in school was very hard and thus when you do the HSC exam it should be pretty easy. And thus a 90/100 as the highest will become your moderated assessment mark, instead of that 40%
check the clink in my sig if you have not alrady read it. it has a good example