The 59 tanks are enough for one tank battalion which, with the addition of two mechanized infantry battalions and some support units, would produce one mechanized brigade. This would be the largest unit Australia would be expected to send overseas. Australia, due to its size and location, does not anticipate being invaded by a hostile armored force. So the M-1s would be mainly for any overseas operations.
There are other reasons for getting M-1s. First, there is reputation. For fifteen years, the M-1 has demonstrated clear battlefield superiority. There is nothing on the horizon that can match it. Then there is the compatibility angle. Australia and the United States are close allies. If Australia gets into an overseas scrape, it will probably be as an American ally.
Now if only Canada would do the same thing.
I don't expect our allies to have full spectrum capabilities that duplicate what we can do. But if enough allies fielded brigades that could plug into our new divisions, they would provide important capabilities. Canada sadly is drawing down to light infantry and giving up their heavy armor even as the value of armor is demonstrated in the war. Canada has stepped up in Afghanistan so I'm not complaining about their commitment--just complaining about their equipment. Australia is stepping up in both categories.