Basically Sharpening the Saw is a vital habit for leaders, Army officers especially. It is not enough to learn a skill once, or to be in peak physical shape once. Our skills and habits must constantly be practiced and honed. If a soldier does not sharpen his saw, he will be less effective to his unit. Marksmanship skills must be constantly practiced to maintain. You have to constantly work out and pt if you hope to attain and maintain physical standards. Those do not just stay with you without putting effort towards it.
But most importantly, soldiers train. They know the basics, but they constantly train to keep that edge gleaming. Without proper training, proficiency, discipline, and skill slips away. Mistakes are made that could potentially be deadly to the unit. Keeping that skill saw sharp is vital to the function of an Army unit. If the basics are well trained, then soldiers can react when things go wrong. And that is an important ability.