Opposing forces may face each other for years, striving for the victory which may be decided in a single day. This being so, to remain in ignorance of the enemy's condition simply because one grudges the outlay of a hundred ounces of silver (to hire spies) is the height of stupidity.
One who acts thus is no leader of men, no present help to his cause, no master of victory. Thus, what enables the wise commander to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge
All calculations in military planning rest on having numbers to work with. Without accurate and broad intelligence on your enemies, you cannot expect to devise effective strategies on how to deal with him. What seems like common sense now was revolutionary then, and took centuries to catch on elsewhere.