Vaults are needed to provide support to the casket so the weight of the dirt and everything on the surface does not apply direct pressure. Vaults also protect the casket from the exposure to the elements below ground. There are different kinds of vaults depending on your needs and budgets.
Some vaults are designed to create a permanent seal from the elements. They are often made with durable plastics and quality structural concrete.
Grave boxes actually allow the casket to be exposed to the elements. These are concrete boxes whose role is to provide structure and support to the dirt surface. These containers can actually have holes in the bottom to allow water to enter and leave the box.