I dare to come back into the fray. Skip is correct, in an perfect environment there would be no need.
Native plants, perfect water, perfect drainage, the periodic wild fire and such and all the microbes that will thrive do thrive in perfect harmony and no one gets sick.
The reality is these products are tools that we can use to management "improved" landscapes on real estate in less than ideal conditions.
I also find it funny that the papers Skip cites do not say they will not help.