The arm ball, or floater, is probably the single best weapon an orthodox finger spinner can have. Ray Illingworth says he once took 41 wickets of 135 in a season just with the arm ball.
At first the ball seems counterproductive. Spinners should spin the ball hard, hoping to impart enough revolutions on the ball for it to dip late in its flight and move off the pitch. This is true for the stock delivery and is what you should do at least 80% of the time.