Zero-Sum Again

I was asked this morning a question from a reader of Trend Following:

“Do you have any references that discuss why the zero-sum concept is critical? The zero-sum concept is interesting theoretically, but seems to have no practical use or meaning. Thanks, John “

I am not sure how you say that after reading every last word of chapters 3 and 4 of my book Trend Following? It is spelled out….especially pages 109-111. I am up for any debate, but not following your foundation here. Hit me back and explain?