Potatoes, sweet potatoes, and taros are examples of starch-rich foods. What happens if you leave them unharvested?　In winter, the stems and leaves will be gone, but the next spring, sprouts will emerge from the same places. This is proof that the potatoes were alive underground during the winter.
The potato needs nutrients to sprout, and it stores the nutrients it made from its leaves during the summer for the next year. The underground potato acts as a storehouse for these nutrients. This stored nutrition is starch. That is why the potatoes we often eat are rich in starch.