Why does stainless steel cookware often have a layer of copper or aluminium on the bottom?

Energy is transferred by thermal conduction. A substance with large value of thermal conductivity is a good thermal conductor and which has small value of thermal conductivity is a poor thermal conductor. The thermal conductivity of stainless steel is 14 W/m.K, whereas thermal conductivity of aluminum is 235 W/m.K and copper is 401 W/m.K. Since the thermal conductivity of copper or aluminum is greater than the stainless steel. That is why stainless steel cookware often does have layer of copper or aluminum on the bottom.