I believe that diagnosis matters in the sense that it leads to the correct treatment. I used to be diagnosed with PTSD and Major Depression and my life was hell all the time as I was on antidepressants which I have since found cause mixed episodes in me. So basically I was left in a constant mixed episode for several years. Not fun. Once diagnosed everything made sense. It took a few years, until this year actually, to find the right med combo but the diagnosis led to that. Without the diagnosis I hate to imagine where I would be now. I still carry the PTSD diagnosis but it is mostly under control and only flares up if I suffer new trauma.