I think diagnosis is important in order to receive the right treatment. You don't want to give certain antidepressants to those with BP and in the case of BPD (one of many reasons pdocs need to distinguish between the two) DBT should be emphasized over medication, though my understanding is that medication in conjunction with DBT can be helpful.

I'm reading all of these 'aha!' moments, and that tells me that it's often important to know. I know it was vital for me: I had a name for what I was experiencing, and I could look back and explain -to a point, anyway- what I had experienced in the past before I was treated (like many here, I was originally, and disastrously, treated for depression and anxiety before being diagnosed, first BPNOS, and then BP I).