The only thing tutoring taught me during my primary school life was that long blocks of learning were boring, truancy was taken lightly and filling out the beginning of computer-read tests were extremely boring. Basically, a complete waste of money since I always finished my tutoring centre's books by copying down the teacher's answers... The only subject I really did myself was Maths - the only subject I found fun... In school my teacher was very good, patient and kind (in my opinion) but I found the work she gave the class on Maths were too simple and I always finished them with ease (I failed to get into OC so I was in the class below it).
From Year 7-10 (and continuing to Year 12) I've had Mathematics Group tutoring at one of the more "prominent" tutoring centres (not mentioning names) and I can say it's helped me a lot in Maths. Just because students get into a selective school does not mean they are guaranteed a high-quality education with teachers that can teach well and/or are willing to help their students... After my Year 7 teacher, I had the same teacher for Years 8-10. He was close to retirement, extremely boring and taught the most un-updated methods for solving Maths questions. Along with refusing to do any work and/or listening in class, the only reason I was able to score good marks was entirely based on... you guessed it - tutoring!
Previous discussions about the "academic pressure" of being behind the other students in the class is entirely true too. Imagine if the class was beginning to learn algebra or right-angled trigonometry and you were struggling, while your friends and the rest of the class was struggling with circle geometry/exponentials or whatever at tutoring.
But then again, it is "common" and "expected" that selective kids go tutoring - it's just a general norm and stereotype that is true. Whether it helps or not is entirely up to the student themself.
TL;DR - Tutoring is and always has been the only thing that has given me "access" to "challenging" Maths - studying ahead, using methods that are simpler and easier to understand than school methods that greatly help me. As a selective student who has had tutoring since Grade 1, if I didn't have mathematics tutoring, i'd be doomed for maths.