If you lose your mental capacity through accident or illness and you don’t have a power of attorney, no one has the legal capacity to deal with your assets and make financial decisions for you, including to sign documents on your behalf.
Your family may have a problem in conducting your financial and business affairs.
In addition, nothing can be bought or sold.
The only solution, if there is no power of attorney, is to apply to the relevant government department for appointment of a financial manager. This may be costly, inconvenient and not what you want.