Your marks are very important, much more than your rank. Marks determine ranks. If you want your school mark to moderate well, you need as many marks as possible, and be close to the highest mark, if you aren't first.
But I don't say this in reference to comparing your percentage to another student at another school. What this is in reference to is how your marks are eventually moderated. Your moderated assessment marks are bounded by the highest and lowest HSC exam marks. Your assessment mark is then determined by how far you are from your peers. Being within 2% of first is better than being within 20% of first, yet you can have the exact same rank in both scenarios.
If you are closer to first, in terms of school marks, this is translate to how close you are to the highest moderated assessment mark. Your rank will not matter in these calculations. You could be in the middle of your cohort, yet since your school marks are close, your moderated assessment mark will be close as well.
As for your marks, they can only be compared internally at your school because they are not standard assessments that all HSC students sit. The only assessment that all students sit is the HSC exam. Therefore it gives rise for BOS to use this to determine the quality of your school and to moderate your school's marks based on your HSC exam performance.
NB: a common misconception is that the 4th ranked student will receive the 4th highest exam mark. This is completely false, and what I have said does not imply this.