...if the only tool you have is a hammer,
every problem looks like a "nail".
- A. Maaslow
What you don't know, you can't do/use/include in your thinking.
This is readily visible in the case of "tools": those items, skills, abilities that you have available to solve problems, accomplish ends, or just plan have fun.
It tends to be very sad when you look the work output from some one, like a web page or just a carpentry job, and you see: if only they knew about X, they would have done a better/faster/cheaper job than they were able to do using those stone age tools.
While the section on "Practical Knowledge" looks at some simple "tools" that people may not be ware of, this section looks at the public results of people using the wrong/inadequate/obsolete tool to accomplish something. The purpose is to learn to recognize the situation so I don't do the same thing (any more than I am aware of! ;-).