Then that person is off for a week.
A newer operator follows the written setup but misses the small adjustment that prevents chatter. Purchasing orders the usual item because nobody knows the alternate supplier. Maintenance spends an hour rediscovering a fault that was fixed six months ago.
Nothing magical disappeared. The shop simply discovered how much of the operating system had been stored in one person's memory.
The goal is not to document every trick someone has ever learned. It is to capture the knowledge that becomes expensive when nobody else can find it.