Why is it considered necessary to nail down the lid of a coffin?

Its not like the corpse is going to run away or anything!Why is it considered necessary to nail down the lid of a coffin?
I heard that the reason was to prevent the corpse escaping due to vampirism. It's an old english religious custom when burying the dead.Why is it considered necessary to nail down the lid of a coffin?
So the body doesnt fall out when being lowered into the grave.
I think also, it may be because sometimes they have to exhume the body and if the coffin wasn't seeled, there wouldn't be much of body left would there? I don't know its just a wild guess.
if the coffin falls the body will roll out. that won't be a nice sight now will it.