An advance health care directive is a document whereby you appoint an agent to make medical decisions for you and provide specific instructions for your health care such as whether to prolong life in the event of an irreversible coma.
A HIPAA authorization is a document where you name people who can have access to your medical records.
In the absence of an appointment of a heath care agent and HIPAA authorization, a person will have to petition the court to either become your health care surrogate or your conservator (also referred to as your guardian). Involving the court can add time and expense to the process of assisting you with medical decisions.
Someone you trust to make medical decisions for you and receive information about your health care. Your health care agent can be your spouse, sibling, child - basically anyone you trust to make these decisions.
It is a good idea to have this so your medical provider can contact them if needed.