A. So they don't fail
B. So they can improve their rank internally or not drop a rank
C. To put themselves in a better position of scoring well (e.g. you can be 1st and achieve a mark of 91, but might want to push that to 94)
That sort of thing...
Judging on your signature there, you'd be pretty happy to get 95 and get into USyd/UNSW Comm (otherwise it's like MCQ/UWS Law?). But if you were to fine tune some things such as essay writing skills in the humanities, improve by about 4 marks in each subject, you could get into Law at UTS (because you can't expect to get 99.7 just because you're paying someone for some assistance each week).
Basically, regardless of how much study you might do on your own and how many times you submit samples to your teacher(s), a third perspective can help in some way. But in the same way, regardless of how many hours you spend with a tutor, you still need to put a lot of hours in on your own accord if you want to see sizeable results.
Check this out if you're interested -
DEREN