A repeatable procedure written down so a task is done the same way every time.
A runbook captures a repeatable procedure — 'how I process the inbox', 'how I refresh the backups' — so the task is done the same way every time.
The test of a procedure is whether it survives you. If "how the backups get refreshed" lives only in your head, the system has a single point of failure with a pulse — and the second-worst outcome is you doing it slightly differently each time and not knowing which version was right.
Written down, it stops being memory and becomes infrastructure: reviewable, improvable, delegable — to a person or to a machine. That last part matters more than it used to. A runbook is already most of the way to a skill an assistant can execute; prose that precise is nearly code.
The honest limit: a runbook nobody runs is worse than none, because it looks like the process is handled. Write them for what you actually do repeatedly, not for the system you wish you ran.