From what woopedazz said, it sounds like, in Australia, internship is primarily separate from the rest residency and applying for a main residency during intern year doesn't put you at any disadvantage. On the other hand, in the US, internship is often times bundled with the rest of residency. So if you try to apply to residency during intern year, most spot have already been filled categorically and there are few advanced spots available. Therefore, oftentimes people need to repeat intern year if trying to switch specialties.