Why use six shafts on an immense battleship?

Why might designers give a Yamato-sized battleship six shafts?

To avoid overstraining individual shafts and to preserve propulsion after battle damage; losing several still leaves the ship mobile.

Six shafts offer engineering and survivability advantages. Distributing power across more shafts reduces the load placed on each individual shaft. It also provides a graded damage response: losing one, two, or even several propulsors does not leave the ship helpless. That redundancy becomes especially valuable when the platform is extraordinarily expensive, strategically important, and expected to survive heavy battle damage.