The company runs inside three people’s heads
Someone knows why that process works the way it does. Someone fixes the spreadsheet before closing the month. Someone solves with a phone call what the system cannot. You know their names. And you know that if two of them leave in the same quarter, the operation stalls.
The cost with no line on your P&LUndocumented critical knowledge is an unprovisioned liability, and it always comes due at the worst possible moment.