This isn't so much about growing baseball in Australia as it is about marketing MLB in Australia. Baseball may not be the biggest sport Down Under, and it may never be the biggest sport there, but it is well established and growing. The Twins have signed a number of players from there, and so far they've not had a lot of impact on the Twins (Grant Balfour didn't get his career going until several years after the Twins cut him loose).
But they have high hopes for Lewis Thorpe, a teen-aged left-handed pitcher who tore up the GCL last summer.
Playing these two games in Sydney isn't necessary to make the game popular there. And it is oddly disruptive; these teams will now have to return to the other side of the planet and pick up afresh a week later.