At disability: If your name is on the title of your assets and you can’t conduct business due to mental or physical incapacity, only a court appointee can sign for you. The court, not your family, will control how your assets are used to care for you through a conservatorship or guardianship (depending on the term used in your state). It can become expensive and time consuming, it is open to the public, and it can be difficult to end even if you recover.
At your death: If you die without an intentional estate plan, your assets will be distributed according to the probate laws in your state. In many states, if you are married and have children, your spouse and children will each receive a share. That means your spouse could receive only a fraction of your estate, which may not be enough to live on. If you have minor children, the court will control their inheritance. If both parents die (i.e., in a car accident), the court will appoint a guardian without knowing whom you would have chosen.
Given the choice—and you do have the choice—wouldn’t you prefer these matters be handled privately by your family, not by the courts? Wouldn’t you prefer to keep control of who receives what and when? And, if you have young children, wouldn’t you prefer to have a say in who will raise them if you can’t?