You need the get decent marks to get decent ranks (well at least I think so) or at least get the best mark out of your class to be ranked number 1. They are just as important as each other. It is a mutually beneficial relationhship. Top mark = top rank. I know this sounds stupid and straight forward but if you don't realise if you get a relatively higher mark in comparison to your class you get the higher rank!
Also, in the HSC isn't half your subject's mark determined by your external examination MARK?
Furthermore, as others have stated; marks allow an individual to forget the strugles and dog-eat-dog world of marks and see if they have met their individual goals (I understand an individual goal can be a high rank, but that is comparing yourself to others). A mark allows you to see how you are fairing in a course, whether your exam technique needs fine-tuning and whether your study quantity and quality needs to be adapted to better suit the demands of a subject.
However, regardless of all this, as you all have said - it is a dog eat dog world where the marks turn into ranks and your UAI shows your HSC performance in comparison to the rest of the state. Also, for your assessment mark (50%) of your HSC mark it is ranks that establish your marks for this, rather than the other way around. So in this respect, ranks are vital.
Conclusion to be made: they are both important - equally important.