The UAI's just reflect supply and demand. The A.C.T is going to have less students finishing year 12 and getting UAI's than most other areas of Australia yet I'd assume the ANU has about has many places to offer as any other university. A lot of people obviously conclude that the costs don't outweigh the benefits or can't be bothered moving to Canberra.
Obviously the rankings aren't the be all and end all, but if ANU staff are producing more from their research from other university staff then you have to conclude that either they are just smarter, or the university has better facilities etc...
Going for a job overseas, the rankings will play more of a role. If an employer has never heard of your uni and looks it up and notices its top 20 in the world then you are going to have a better shot at the job.
Also the thing about courses like optometry is that they generally have a lot of vacancies at less prestigious universities, someone who doesn't know what they want to do or missed out on the course they wanted isn't going to say "oh look i can try get into optometry i'll do that", where as they might do something like arts.