An advance directive lists what kinds of treatment you would want if you become seriously ill or unable to make decisions for yourself. This document legally designates who your surrogate decision maker, also called a proxy or agent, will be. This person is your effective durable power of attorney for health care decisions.
Ethical practices and the law support the right of the patient, or their surrogate, to decide which treatments or procedures they will have (or not have). These laws/ethics are based on the concept of autonomy. The law that reinforces this is called the Patient Self Determination Act (1991).