It's really as simple as that. Because that is where your vote is going to end up in the House of Reps election, one way or another.
You can put them last, second last etc. but no matter what you do, under our preferential system unless your candidate gets a huge primary vote, it will almost certainly go one way or the other to one of the big 2.
In the Senate, you have the right to number all your votes right down to the last number, to control exactly where your vote ends up.
So choose wisely my friends, because the future of our nation depends on it!
