As a student that attends a school that does not foster an academically competitive environment, I would know the difference that tutoring centres have made on my educational journey to a great extent. I don't attend an academically selective school, meaning that efforts placed on rigorous training and coaching is very minimal.
My parents have invested a lot of money into tutoring centres such as NorthShore, QED, Dux College, Matrix Education, ACE, Dr Du and many more. Some were great, while others were not providing the coaching that I really required. However, after all this, I can surely state that tuition is an investment if you do not attend an academically selective school. With no motivational peers, and an environment that focuses on other aspects of school life other than the competitive nature of the HSC, one can only receive top grades by being around students that DO care about their studies -- and most of these students can mainly be found in tutoring centres.
Remember that the HSC is not a race that the student must go through alone. Its is a family effort. A communal effort. Everybody plays a part in your final outcome. The educational environment that surrounds you can definitely change the grades that you get.