This means the submarine can be kept at sea for over two thirds of its operational lifetime. An SSBN at sea and submerged is essentially invisible, and hence survivable from an enemy attack. This enables proper execution of the SSBN's strategic deterrence mission. Each crew must be ready at all times while on patrol. They must be experts on their equipment to keep it running during the long patrol.
Intensive instruction serves to continually upgrade their knowledge.
Finally, there are refinements and improvements consistently being made to the weapons system's various equipment operating procedures. On a fleet ballistic missile submarine the enlisted crew is made of technical experts from different occupational fields. These personnel ensure the ship is self sufficient while at sea. A submarine at sea is pretty much a small town with all the equivalent jobs being handled by the crew. Continuously monitor, troubleshoot and repair electrical equipment.