Why have both a thermopile and thermocouple?

A thermopile generates electricity from the heat of a pilot light. It generates about 750 mV that turns on the main gas valve to the fireplace.

A thermocouple is heated by the pilot light but it uses pressure to hold the pilot light gas supply valve open. When the thermocouple is heated, pressure increases holding open the main gas valve. If it cools down, the pressure goes down and closes the gas valve.