To give the world champions a chance to win the game of course.
Edit: I actually have no idea. They seem to change the rules of rugby every week, a bit like the handball rule in football.
Because an advantage in football isn't really an advantage at all. When the team squander their opportunity after playing on, everyone moans that the ref didn't give the foul. Rugby have it correct as their way is a proper advantage. It's a very good rule/law.