Lawns require fertilizer to remain healthy. Proper fertilization practices will lead to a thick, dark green, uniform lawn that is competitive against weed and disease invasions. The nutrients contained in fertilizers are necessary to support many processes occurring within the plants. If any essential nutrient is limiting, the plants will not perform at their highest level.
Munshaw, Gregg, "Fertilizing Your Lawn" (2014). Agriculture and Natural Resources Publications. Paper 135.