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

Well it stems back to olden days when people were afraid of zombies. It was meant to keep the dead from rising after they were pronounced dead. Now its more a ritualistic thing. They also used to cut off peoples heads to keep them dead.Why is it considered necessary to nail down the lid of a coffin?
To thwart (Or at least discourage) body-snatchers.Why is it considered necessary to nail down the lid of a coffin?
Soo the people wont come back out .duhhhh
To slow down graverobbers.