A certified copy of some documents may be required for official government or court purposes and for commercial purposes. Examples of document copies that may need to be certified include living wills, powers of attorney, medical powers of attorney, wills, trusts, titles, deeds, and certificates for stocks/bonds/investments.
Part of preparing for a disaster (whether it be flood, fire, earthquake, tornado, or hurricane) includes disaster-proofing your important documents--that is, making sure that after a disaster you have the information and documentation necessary to speed the recovery process. Preparing in this way also means you can focus on personal safety when it counts, and not worry about gathering documents at the last minute.
The EFFAK put together by FEMA and Operation Hope is an excellent source to guide individuals and families as they collect, organize, and protect critical financial, medical, and household contact information. This FEMA handout also includes interactive forms to help you prepare your EFFAK.