If you can’t conduct business due to mental or physical incapacity (dementia, stroke, heart attack, etc.), only a court appointee can sign for you — even if you have a will. (Remember, a will only goes into effect after you die.)
Once the court gets involved, it usually stays involved until you recover or die and it, not your family, will control how your assets are used to care for you. This public, probate process can be expensive, embarrassing, time consuming and difficult to end. It does not replace probate at death, so your family may have to go through probate court twice.