By doing this you appoint someone to make the medical decisions when you can't.
The focus is to make your wishes clear, to tell someone about them, and importantly to do it while you are clearly competent.
But first get the background by listening to this podcast:
Who decides how we die?
‘advanced health care directive’"advance" or "advanced" it is still a directive that is made well in advance of the need for it.
It's a Living Thing