When I was a kid, few things were as fascinating as using a magnifying glass to focus the rays of the sun to a pin-point.
We operate under several constraints. Our time, energy, attention, will-power, productivity are all limited at a given instance. It is possible to run fast and burn through those resources at tremendous speed without really getting anywhere. The most successful people are the ones who can focus them around a clear, central purpose.
Ask somebody who wears glasses, and they can tell you the difference between what is clear and pretty clear. Unless the lens they wear is manufactured to perfection, vision is blurred.
A central and essential purpose is the difference between blurry and 20:20 vision.