According to Guns, Germs, and Steel, why was the first occupation of Australia important?
There are many possible answers to this. However, the most likely answer is one that can be found in Chapter 1. In that chapter, we are given two reasons that the occupation of Australia by human beings was important.
The first reason is because the occupation marked a first in human history. Australia, even at the time when sea levels were lowest during the Ice Age, was not accessible by land from any other part of the world. Diamond says (on p. 41 of the paperback edition of the book) that to get to Australia (which was connected with New Guinea at that time)
…required crossing a minimum of eight channels, the broadest of which was at least 50 miles wide.
What this means is that the people who occupied Australia had to have come by boat. It is the earliest evidence that we have of people crossing significant distances of open water. This shows that the early Australians were not technologically backwards.
The second reason is that the first Australians seem to have killed off all of the large animals in Australia. Not everyone agrees with this, but Diamond argues that human occupation led to the extinction of all of the megafauna on Australia.