If you wear glasses, you understand focus really well. Why is it easy for people who need glasses? Because many of them just cannot see what is in front of them without their glasses. If you cannot see clearly, it is very hard to function effectively.
I have been shortsighted since I was 15 years old. Without my glasses, I cannot read well, drive, enjoy TV or notice facial expressions from half a meter away. Without glasses, I cannot do my presentations, coach, cook, clean the house or even choose what clothes to wear. My vision plays a big part in my life, and without it, I am crippled.
Mental focus works the same. If we are not focused, it is as if our vision is blurred. The lack of focus affects all aspects of our life negatively and we are crippled.