Here's why:

* Nutrition is the process by which organisms obtain and use food for growth, repair, and energy.

* Photosynthesis is the process by which plants and some other organisms use sunlight, water, and carbon dioxide to create their own food (glucose) and release oxygen. This glucose is their source of energy and building blocks for growth.

Therefore, photosynthesis allows organisms to produce their own food and is therefore a key part of their nutritional process.