The problem
When someone leaves, the company loses more than a person. They lose the knowledge that person carried.
Which customers they were managing. Which suppliers they had relationships with. Which orders were in progress. Which tasks were left unfinished.
The standard response is to ask the person to write everything down before they leave. Sometimes this happens. Often it does not. And even when it does, the document sits somewhere and nobody knows where to find it.
The next person starts from close to zero. It takes weeks or months before they are fully up to speed. During that time things slip.