Almost everything a household knows can be rediscovered with a few phone calls. A small number of things cannot, or can only be rediscovered at real cost and on a bad day. Those are the ones worth writing down.
The expensive-to-reconstruct set
Account numbers and providers — utilities, insurance, mortgage — with the phone number that reaches a human. Serial numbers for anything under warranty. What was replaced in the house and when, especially roof, heating, and anything behind a wall. Medical basics: blood types, allergies, current prescriptions, and which doctor. Where the water shutoff and the breaker panel are.
And the one people always skip
Who to call for what. Not a directory of businesses — the specific plumber you already trust, the neighbor with the spare key, the relative who should be told first. This is the list that is worthless right up until the moment it is the only thing that matters.