Why is the percentage of food eaten a good
number to use?

The percentage of food eaten will let us assume about the availability of food for organisms of a species. If a food percentage happens to be high, then we can say that particular species is successful. In contrast, if the food percentage is low then the population of that species will shrink. There will be competition for food among the member of the species and this will cause the species to become unstable in an environment.

because the light is a great source of energy for the plants to survive. (i hope this )