Purpose of a ball joint in the steering system?

On vehicles, the ball joint(s) connect the steering spindles or struts to the control arms and allow for limited horizontal and vertical movement of the spindles/struts, as well as allow the spindle/strut to turn on a horizontal plane so that the vehicle can steer.

ball joints deviate velocity due to up and down motion.

The ball and socket joint consist in the hip and shoulder. It is a spherical bone fitted into a cup like bone for maximum movement.