The problem with a rugby ball’s oval shape is that it produces a lot of drag when it travels through the air. This can make it wobble and slows the ball down. That is why good players spin the ball when they throw it.
Spinning a rugby ball keeps it lined up in the same direction as it was thrown, as it flies through the air. It stops the ball wobbling as it produces less drag, which means it will travel further and has more chance of reaching its target.