There are many reasons, and here’s a good one to start with:
Avoiding harmful chemicals. There are many toxic chemicals in and on conventionally produced foods, including fungicides, herbicides, pesticides, and synthetic fertilizers. These chemicals are fat soluble: that means that these toxins are stored in our fat cells, and accumulate over our lives, contributing significantly to illness. Organic foods to not use these chemicals.
To learn more, check out the site What’s On My Food?