Diagnosis matters because it determines proper medication and psychotherapeutic treatment. That's why MDs and therapists have to be in agreement. They cannot work at cross purposes. It's stressful on patients and the outcome is poor. MDs have the upper hand in determining acceptable diagnoses, particularly involving medication. Some people do Dr. shop to get a diagnosis they find desirable. I truly don't know why anyone would actually want a particular diagnosis, but it happens.

Think of it this way, would you walk into your primary care doctor's office and not care if you had pneumonia or bronchitis? Like wise would you walk into your primary care doctor's office and insist he diagnose you with a brain tumor when you actually have a sinus infection? The correct diagnosis really does matter.