Why is the lid of a coffin nailed down?

So during a zombie apocalypse the zombies can't get out.

So when tu wake up tu cant get out. lol, I have no clue. Probably something about bugs and things getting in, but then again that wouldn't help much in the long run anyways.

Idk, just in case it floods o something and will help the body not just fall out...

To keep zombies in.