Why Germany moved from three shafts to four

Why did German warships move from three propeller shafts to four?

Four shafts distributed and controlled high power more effectively while providing a measure of propulsion redundancy.

German capital ships moved from three shafts before the First World War to four in later designs as machinery outputs increased. The comparison is similar to distributing automotive power across more driven wheels. Four shafts make it easier to put a larger total output into the water in a balanced and controllable way. They can also improve survivability because damage to one shaft removes a smaller share of the propulsion system.