Seriously, disasters happen. People go AWOL, get seriously ill, leave without notice, get caught embezzling and are escorted out that day, etc. How screwed you will be when that happens depends largely on how well you plan ahead to minimize the impact of such disasters.
In my office, we call it the "hit by a bus" plan. The idea is to document enough key information that if someone gets hit by a bus tomorrow, their department would be able to continue functioning. (We're a sensitive bunch.) This means that information related to the job is all written down in a formal manual, not just recorded in someone's head.
Each staffer is responsible for keeping his or her own "hit by a bus" manual up-to-date with information about contacts, passcodes, procedures, notes about key non-staff personalities that they interact with regularly, etc. It's included in everyone's job description and it's one of the items we evaluate managers on when we do annual evaluations.
I cannot recommend it highly enough. There's enough chaos when you unexpectedly lose a staff member; you will be relieved not to be scrambling for these basics.