The brain that doesn't retire.
Your VP of Operations has 18 years of context in his head: customer histories, supplier relationships, why the $200K risk clause exists, who to call when the system goes down. He's retiring in 8 months. The replacement starts from zero.
Or: your senior account manager just got promoted. The customer accounts she ran have three years of history in her brain. Her replacement is calling you every morning asking "do you remember what happened with..."
The business doesn't run on what it has documented. It runs on what people remember. Until they don't.