Without going into a lot of detail, I think a diagnosis is important. For years I wanted to know what was wrong with me. Why am I the way I am? Once I had a name for it I felt a sense of relief! There was a name for the madness I was suffering from. I did my research about the disorder and that helped me to understand my symptoms. Ever since then, I feel like I have a better 'grip' on this illness so that i can get treatment when I need it.
I also think having the diagnosis is for billing purposes for the doctor.