So essentially, your assessment marks don't count towards anything...? Does it mean your assessment marks mean nothing? Wow. I feel totally robbed.
Your assessment mark is crucial as it shows your rank AND the gaps between you and those around you.
E.g two students have an assessment mark of 90 at their respective schools.
That mark alone is meaningless as we don't know where they are position wise or how hard or easy the tasks were but after the exam the first student earned an exam mark of 94 and the second an exam mark of 70 we get a bit more information but still not a complete picture.
Now say that the first student came 2nd at their school by 3 marks and was also 4 marks behind 1st on the exam we get to see a student who is very close to the top and has also done well on the exam. They keep their exam mark of 94 and the BOS will lift their assessment mark to be a reflection of their 2nd place ranking and their 4 mark gap (but due to the fact that schools use 0 - 100 and the BOS largely uses 50 - 100 the mark will probably shrink to 3 or even 2) giving them an assessment mark of 96 with a final mark of 95.
But student 2, whose school was a bit easy with its tasks hasn't done as well on the exam and nor has their cohort was also ranked second but was 8 marks behind first. They will keep their 70 of course for their exam mark bit their assessment mark will be determined by the top mark (which for my discussion is going to be 89). This will see the second students assessment mark come in around 84 or so given the aligning process which takes most marks to between 50 and 100 leaving them with a final HSC mark of 77.
Now you can see that the assessment mark isn't of itself important but it is important in determing how many marks you will get.