Why is the lid of a coffin nailed down?

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

So when u wake up u 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 of something and will help the body not just fall out...

To keep zombies in.