How would you go about treatment for illnesses that have no known cause?
There are illnesses where we have a known set of symptoms, a known progression, and known treatment options, but have yet to discover the underlying set of causes. You may be able to reliably diagnose, and treat, without understanding the mechanisms at play.
You're not going to be able to work with the bureaucratic side of the healthcare system without being able to say what they're being treated for. Those treatment options can be approved for one illness and not another, even when they share symptoms, because of known efficacies.
Having a name to call something helps, even when you have no idea about the mechanism at play.