It isn’t really a “Will” (which is aimed at regulating your estate only once you are dead), but rather an “advance directive” to doctors, hospitals and your family as to what end-of-life medical treatment you do and do not consent to. 

Whilst the validity of a Living Will cannot be guaranteed (there being still-untested legal considerations involved here), it should at the very least make it easier for those caring for you at the end to make hard decisions in need.