People are concerned that when they pass away or are mentally incapacitated that their family will not be able to track down all their financial information. It is normally recommended that you keep all your information in a notebook or similar spot. This is hard to keep up to date and with so much of our life being transferred online it is difficult print it out and then get it into a notebook.
A shortcut which will capture a majority of your financial information is your yearly tax return. If you keep all the documents such as the 1099s and W2s with the tax return, your family will quickly be able to track down your assets. If you have a bill for your safety deposit box, throw that in there also so they at least know what bank has the box.
A fallback is to pull these records from the IRS transcripts. They will need to request an IRS specific Power of Attorney, provide the IRS with a Letters Testamentary from the court if they are deceased or a Durable Power of Attorney is they are alive.