Why is open pit mining is used?

like strip mining open-pit mining involves removing the surface layer of soil. Miners then use explosives to break up the underlying rock and recover the ore. As they dig a deep hole or pit to mine the ore they build roads up on the sides of the pit. Trucks carry the ore to the surface. Ores of copper and iron are obtained by open-pit mining. (read in earth science book by Mcdougal Littell.)