There are plenty of diseases we can detect but not, yet, treat. There are other uses for such knowledge. The diagnoses person can adapt their lifestyle to accommodate the inevitable disease progression. That directly aids the individual. And knowledge of the person's inevitable decline can be used by the rest of society. Children can make necessary arrangements. Safety measures can be put in place, preventing unnecessary injury to the individual and others. If someone has been diagnosed with an inevitable rapid onset of Alzheimer's, they probably shouldn't retire to a cabin atop a mountain accessible only by helicopter.