I believe there are some treatments out there aimed at slowing down the progress of the disease. Diagnosing early might mean slowing down the nasty stuff earlier, thus improving the life of the patient. Also, this sounds like something that is not at all part of the known symptoms of disease. Imagine if the change of smell is linked to something that is a cause for parkinson (I'm not suggesting the article even considers that option, but it's not impossible), and imagine that that something is actually "easy" to treat for. That would change a lot of things ! Yes, it's very unlikely, but even a small increase in the knowledge of a disease is always progress.