Why do venus flytrap need to eat insects?

The Venus Flytrap, along with other carniverous plants, evolved in an environment devoid of nitrogen and phosphorous compounds which it would need to survive. It obtains these "foods" by digesting prey, whereas most plants obtain these foods from the soil via their root systems.
Flytraps actually get a good deal of their sustenance like other plants do, through the process of photosynthesis. During photosynthesis, plants use the energy of the sun to drive a reaction that converts carbon dioxide and water to sugar and oxygen