Michigan law refers to Durable Powers of Attorney for Health Care; I prefer the term, "Medical Powers of Attorney", because it is simpler, and it readily distinguishes this type of power of attorney from a financial/business power.
In a Medical Power of Attorney, you can designate, not just who will act on your behalf, but a mechanism for decision-making as well. Perhaps doctors or family members disagree as to the best course of action, a properly drafted medical power of attorney can provide guidance to medical staffs and worried family members, as to how best to proceed.
Perhaps most importantly, you can make sure that medical decision making is made in accordance with your own personal and religious preferences.
While some people readily say that they do not want to live where doing so would require long-term hospitalization or heroic measures, others' personal or religious views prevent them from doing anything that would terminate a life that would otherwise exist.
We all recall the tragic case of Terry Schiavo, a few years back; Mrs. Schiavo was a woman in Florida, whose condition may or may not have been persistently terminal. You may recall it concerned the issue of whether life support should be continued, and it involved a situation where the woman's husband disagreed vehemently with Mrs. Schiavo's family as to how to proceed.
My point in raising this subject is not to voice an opinion, as to who was right. If I were forced to offer an opinion, it would be this: both sides were right. All views on this subject are important, everyone's preference should be respected.
A properly drafted medical power of attorney will ensure just that. In turn, the respect given to clients' faith-driven and/or personal wishes can be a great source of comfort in a difficult time.