So they need to make sure they had enough runs on the board to force Aus to bat agressively enough to have to score runs to have a chance of winning so they had more chance of taking wickets. The other side to that coin is, if they set too high a total then the chasing side could look at it and decide it's too much and just play defensively for the draw, making it hard for the bowling side to take enough wickets to get them out and win.
It's a subtle game cricket.
Sums it up, Ozz could not beat us, so they went for the draw, either way, that rule is stupid!
Look, here's the most simple and straightforward explanation of cricket:
You have two sides, one out in the field and one in. Each man that's in the side that's in goes out, and when he's out he comes in and the next man goes in until he's out. When they are all out, the side that's out comes in and the side that's been in goes out and tries to get those coming in, out. Sometimes you get men still in and not out.
When a man goes out to go in, the men who are out try to get him out, and when he is out he goes in and the next man in goes out and goes in. There are two men called umpires who stay out all the time and they decide when the men who are in are out. When both sides have been in and all the men have been out, and both sides have been out twice after all the men have been in, including those who are not out, that is the end of the game.