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

In case they're not quite dead! or maybe to stop them falling out if it's dropped.Why is it considered necessary to nail down the lid of a coffin?
no idea...I could say it dates back to medieval times when it was customary to spin the coffin three times...but of course I would be lying.Why is it considered necessary to nail down the lid of a coffin?
Because there might be a Politician / Banker in it
Just in case the occupant changes his/her mind....
Grave robbers!