Answer: because this has a relation with the food chain of lower levels like herbivores.
Explanation:
as you know plastids are the main source of food production in plants. As they meet the food requirements in plants but animals don't have it so the lower levels like deer who eat grass depend on these plastids of plants to fulfill the need for food and nutrients as animals cant process or made their food. and carnivores fed on these herbivores for their food consumption. so you can indirectly relate this statement to the food chain. and this is how animals depend on plastids found in plants.
