Well, it's a degree aimed at giving undergraduates some major research experience, which is very unusual. Most degrees do not offer any research component (except of course the optional honours year, because a honours thesis should qualify as research). So the only way to obtain any research experience is via summer research programs (which many universities offer in varying topics) and industry research-based internships. The PhB program has the additional benefit of giving you six semester-long advanced studies courses, or ASCs for short, which introduce you to the world of academic research. Having a research-based background is hugely advantageous in applying for PhDs, where the whole point is to present yourself as a person who can contribute novel academic research via a future PhD thesis. That being said, this is only particularly important when applying for top universities in the US or UK, or applying for exceptional postgraduate scholarships - you can get into any decent PhD program in Australia, and be awarded the standard scholarship (APA - $23000ish per year) provided you achieved first class honours in some (good) undergraduate degree.
Furthermore, some ASCs can lead to further collaborations with academics, which in turn can lead to major publications and/or presentations at academic conferences. It's these kind of experiences that really stand out on a resume. (Note that this depends very much on your area of study - it's not particularly feasible in mathematics, but I've had friends in experimental physics with these experiences.)