Whatever role you are in, it's important for everyone around you to have an idea of what you do and how they could replace you in case you were hit by a bus tomorrow. It doesn't even have to be such a permanent situation as that. If you broke your leg and had to stay home from work for six weeks, how would the business cope? In some cases - menial work - there won't be much of a problem. Someone else will cover your shift, some other person with similar skills can fill in on a temp basis, no big deal. In some other roles, though - husband, parent, museum curator - your disappearance or disablement may cause serious disruption to the people around you. If you are proactive about this, you should prepare for this eventuality. I call this "the black envelope".
Inside the black envelope is everything anyone would need in case you are no longer available to fill your role, permanently or only temporarily. What you do day to day, how you do it and where everything important is kept. It's like putting together a job handover package before it's necessary, because you might not get to deliver all the information in person.
Mokalus of Borg
PS - I actually don't have one of my own yet.
PPS - But, like backups, I've been meaning to start any day now.