The unfortunate reality is that life could take a turn that makes it impossible for you to communicate your medical wishes to your family or healthcare professionals. You could suffer a stroke or get in a car accident, for instance. You still want the doctors to know what type of care to provide, and you can use an advance directive to give them instructions.
I was working with a client recently who had just gotten divorced. He wanted to update his estate plan to make sure that all of his ex-wife's rights to his estate had actually been terminated. Naturally, if he passed away, he did not want to leave her his money and physical property. He wanted it to go to his children.