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

So the body doesn't fall out if you drop it.Why is it considered necessary to nail down or lock the lid of a coffin?
First, one wants to be sure that the lid does not come off during transport, or if a pallbearer drops it (don't want anyone falling out). Also, if the person is being buried with a specific item, especially those of value, less chance of it being stolen before or after burial if the lid is secure.Why is it considered necessary to nail down or lock the lid of a coffin?
if there is a flood and the coffin is raised to the surface for any reason, it would keep the body from being exposed.
maybe to make it more difficult for grave robbers?
So If the person wasn't dead, he wouldn't have the chance to come back to his loved ones, who miss him/her terribly.