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

the stainless steel cookware does not spread the heat evenly , therefore to spread the heat a copper bottom is used  so that  the food is cooked effectively and feasibly.

 

Since the material does not adequately spread the heat itself, stainless steel cookware is generally made as a cladding of stainless steel on both sides of an aluminum or copper core to conduct the heat across all sides, thereby reducing "hot spots", or with a disk of copper or aluminum on just the base to conduct