Why, I will tell you why. Get ready for a long lecture young one.
First of all, I will show you the relationship between HSC Mathematics content and Preliminary Mathematics content. Now, this is what complacent people say, "30%". What does this mean? Oh great we only need to know 30% of the Preliminary course to do well in the HSC. No, you must be strong with all your topics. If you are not strong now, chances are you will be forced to revise next year and you do not want to be doing that. You must be very strong with your topics, if you are not at the moment, that's fine, you still have time to change.
But it's the ability in Mathematics to consistently find links within questions and those patterns that makes the art of Mathematics appreciable. So my last piece of advice to you human is to learn everything. The art of study is like shopping, you need to compare things, the cost of it and the effect on other predicaments. Now Mathematics should be appreciated by everyone. There is nothing better than seeing little ones learn this art. There is nothing better than being stuck on a question and hours upon hours trying to do it then finally getting it. Truly rewarding.