Why to use bedding under storm water drains?

bedding under storm water drainis used to provide a steady support of the drain and minimize deflecion and or movement of the pipe, even if water enders the area beneath the drain, that is why rock or thick gravel is used for proper drainage under the pipe.

To controle any setlment beneath the bedding and to protect the pipes from  deflecion or movement and the effictive of directe loads over the pipes ,,, thankyou