The difference between first and second place in a race is often less than a second. In golf, it may be an inch on a putt that makes the difference between the winner and everyone else. Yet the reward for being just slightly better is often several times as much as it would be for coming in second. Reading the green correctly and making that extra one inch on a putt could mean hundreds of thousands of dollars.
The little things, the details, are so crucially important in life. In the Army, those details can mean the difference between life and death. The term "Attention to detail" can be heard on any American military post several times a day instructing someone to remember to pay attention to the little things.
Every shot when qualifying on your weapon could mean the difference between being promoted that year or having to wait until the next year's promotions come up. Small mistakes can be career-enders when you are dealing with large equipment, deadly explosives, and complex machines. In artillery, moving a howitzer to the left just one degree would make the round land 500 meters to the side at max range. Not properly clearing a weapon could lead to a negligent discharge and kill a fellow soldier.
Oftentimes it's not doing the big things right that really counts, it's doing the little things right.