Well, it is just another hurdle, for somebody who has successfully completed HSC in a manner that expresses competency to study law, it is just more "red tape," so to speak that is designed to filter people who would not necessarily struggle to study law (in other words, it can be very indiscriminate)...
Another thing,
I would liken these tests to psychometric tests on steroids... These test do not necessarily prove or disprove that you are capable or incapable of doing well at Law... In fact, ALL the questions have nothing to do with Law... The tests can also be disheartening, they are quite a lot of people competing for a position which can have a negative impact to your test performance, there are about 2000 people at any one time doing these tests in Victoria (this is particularly negative towards those who may have low self esteem, etc)...
Another side note, although these psychometric test can indicate cognitive capability (the results also depend on the performance of others taking the test, your performance is also dependent on how other do, not on how well you actually perform... You could be dynamite, but if everyone was an A Bomb, you would do crap), they are not the be all and end all to decide whether a person is capable... I recently did one for a job I am applying for, and there is a collection of disclaimers which delve into this (in other words, they may be an indicator of competency but they are not necessarily a good indicator)... It seems that the makers of these test do not even have faith in the tests...
A better way of doing things would be to probably interview candidates to personalise the process and assess competency... The test is just a cop out because it is easier to organise, cheaper and quicker (everything in a Law School seems to be money motivated in this day and age though)...
Moral of the story, if you want to do well at these tests, give everyone gastro prior to the exam, thin down the talent pool a bit...