In ideal conditions (good school teacher, good study habits of student), no tutor is needed.
Sometimes, the student is unfortunately in a non-ideal condition. I have tutored students from top “branded” IP schools who encountered the following:
– School teacher didn’t teach essential math concepts/formulae that are subsequently tested in the exam.
– School teacher didn’t have time to cover the topics. As a result, they only finish the syllabus one week before the exam. (Or worse, did not manage to finish the syllabus before the exam)
– Too hectic and busy CCA training/competitions. As a result, the student did not build a solid foundation during early years of Secondary 1-3, and is only seriously studying full-time in Secondary 4 (similar for JC Year 1 vs Year 2).
The above points can’t really be controlled, it is sometimes a matter of luck whether the school teacher can teach well or not. In unlucky events such as above (which is quite common actually), a tutor is recommended.