My understanding has been that casuals are a temporary solution to fill gaps in the various sectors of employment

There are probably some situations where that applies, but I'd bet it's actually very rare. It costs a lot of money to hire someone, they need to be trained/etc and a lot of that burden on the employer is the same whether you pay them 1 hour a week or 40 hours a week. I'm pretty sure employers want each individual employee to work as many hours as possible - you will always get better productivity from one person working 40 hours a week than from ten people working 4 hours a week. It costs exactly the same in wages, but the output is better with longer hours (up to a point - once people work overtime there are burnout/etc issues, and productivity drops).

There is no limit on the number of hours a casual employee can work, and if at all possible most employers would like that number to be 40 hours a week just like full time staff. A lot of "casual" staff do work 40 hours a week (I do).

Full time employees are entitled to 6 weeks of paid leave (combination sick pay and holiday) every year.

Casual employees who work "full time" are not entitled to any paid leave at all.

As far as I know that is basically the only difference between the two. Another commonly cited reason is it's easier to fire a casual employee but that's only really true if the casual employee is not working regular hours. If they're rostered on regularly they get the same protections as a full time employee.

The other difference is hourly wages are (or at least should be) higher for casual employees, but your annual pay is (or should be) exactly the same if you take six weeks of "unpaid" leave.

My employer gave me a choice, and I chose casual. I like the extra hourly pay - six weeks is a lot of money and it earns quite a bit of interest (especially these days!) in a savings account waiting for my next holiday. It also encourages me to go on holidays where I don't spend much money. Camping for example.

It is easier to apply for loans/etc if you're full time... that's the only benefit I can think of.