It's exactly as Ash says, it put's the ball joint in the centre of it's movement, there is a possibility that the joint would "lock" at the end of it's angular travel without it. The joint might also wear more quickly as the ball would be constantly wearing one side of the socket.
Incidently it's probably good practice to replace the joints periodically, 15,000 miles or three years, worn joints can produce odd handling.
Cheers
Chris
