In most areas of the country, no state or local law require you to buy a container to surround the casket in the grave. However many cemeteries require that you have such a container in order to maintain the grave. Either a burial vault or a grave liner will satisfy these requirements.
The descriptions “Protective” or “Airtight” are the manufacturers’. Neither the funeral home nor any of its employees represents or implies that any vault will be air-tight or will provide long-term preservation of human remains.