What exactly is the manifold insulator meant to do?

The manifold insulator keeps the intake air cooler resulting in more power gain. A cooler intake increases air density increasing the capacity for power gains and power consistency. Furthermore, the insulator increases low-end torque due to an increased runner length. http://www.maceengineering.com.au/epages/shop.sf/en_AU/?ObjectPath=/Shops/mace/Products/MI607