I have this theory that they developed the Search Ads interface without any concept of a promotional credit, just an invoice was raised weekly and your card got billed, and someone decided at the last minute the best way to promote this and get it going was to give everyone some credit up front. Because it wasn't designed for tracking credit in an account as such (as you weren't supposed to have an account that could hold credit), the interface has never been built that way. The whole thing feels a bit like what putting your own ads through iAd used to be like, so maybe they've reused chunks of that system for this.
I have a free app I promote using this as well. It's useful for getting exposure on the App Store and getting to the top of the list as far as users are aware once in a while, but it has to be seen whether this can be used to drive you up the rankings far enough that when you stop you don't have to use it anymore. I suspect that it probably won't work out like that. That's one of those decisions you have to consider depending on what your whole marketing strategy and budget is at the end of the day. But it's definitely easier to measure any financial benefit when used for paid apps as there's a direct 1-to-1 relationship between money out and money in.
In your case I'd assume you have another 30 AUD, set the limits based on this, and if you end up spending a small bit of your own money, or don't entirely use every last cent of the credit it's not a disaster as it'll only be a $ or two either way.
The other thing I've not mentioned above that's useful for free apps, is that you finally get some hard evidence on the keyword combinations users use - handy for ASO purposes at the very least. That's if you're running any Search Match or Broad Match campaigns and the keywords are searched on often enough to escape the 'Low Volume Terms' trap (look over the entire history of your campaign on the Search Terms tab to maximise the number shown).