The idea
All other ununtentional killing is coverered by the Manslaughter legislation where conviction and sentencing takes into consideration the circumstances leading up to the death. I do not think that the fact that the 'weapon' was a car, rather than, say, a knife or fists should make it a different crime.
Why is it important?
Having separate legislation for causing death with a car can lead to the perception that this is less serious than other types of manslaughter.