People sitting in back seat should use safety belts for the same reasons they should use them in the front seat: to reduce serious injuries and fatalities in a crash. Lap and shoulder belts reduce the risk of fatal injury by 44 percent among back-seat outboard occupants in passenger cars and 73 percent among back-seat outboard occupants of vans and SUVs. In a frontal crash, drivers and front-seat passengers are at increased risk of injury from unbelted back-seat passengers, and in a side-impact crash, passengers sitting adjacent to unbelted passengers are at increased risk of injury. Exposure to unbelted occupants increases the risk of injury or death to other occupants in the vehicle by 40 percent.
via IIHS-HLDI.