The digestive system is the group of organs that break down food in order to absorb its nutrients. The nutrients in food are used by the body as fuel to keep all the body systems working. The leftover parts of food which cannot be broken down, digested, or absorbed are excreted as bowel movements (stool).
I believe the answer to this is it creates nutrients for the plants. Light can be another way of saying sunlight (solar) and with sunlight it helps the plants get the nutrients they need and they can grow.