„Mary’s been with the company forever. She knows where all the bodies are buried.“
This means that the company management had better be very, very nice to Mary.
Knowing where „all the bodies are buried“ means knowing all the company’s secrets. Every last one of them. And that means power!
Why? Well, no matter how well run a company is. No matter how honest or how ethical. There will always be one „body“ or another that had better stay buried.