Why farmers may want to keep, not kill, weeds

I took a class at UC Berkeley around 1978, called Weeds. It was taught by Boise Day, who belonged to a group of professors I referred to as the plant pathology cowboys. In one class, he drew our attention to a table in our text that demonstrated the nutrients  calculated from a corn field and those of the weeds in that cornfield.  The weeds were obviously winning, and the prof asked us, so what does that mean to us? I replied, that we should be  eating the weeds and not the corn. He did not appreciate this point of view.