Very, it was even important when the diagnosis was wrong (Autistic Spectrum Disorder), but now I know that I have the correct "label". Even now I still have this though as I have been told I have a dissociative disorder (which I knew anyway), but even though it's been talked about since, nothing has come of it. Also I've been told by my therapist I have all of the traits of someone with Avoidant Personality Disorder, but she's hesitant to diagnose me because she thinks as I heal, that will too -though that doesn't mean I don't have it.
Between the two it's incredibly irritating (and slightly invalidating) especially when bouncing around between hospitals, GPs and Mental Health Teams. I can't just say: I have Depersonalisation Disorder, PTSD and Avoidant Personality Disorder because that won't be backed up by my file, (hell my GP doesn't know any of it because it got lost in the mail and is confused by me) ,instead I have to try and explain it to people who are too busy to care enough to listen and don't know me well enough to not jump to their own conclusions.
Some of these have been distressing, for example being told I have nihilistic delusions, when my issues are not psychotic but dissociative isn't helpful. Nor is being told, I must have Borderline, because I self harm - completely ignoring the fact I have PTSD because apparently the additional dissociative symptoms confuse them. Or being blamed by the hospital for not being able to do certain things (mostly socially) that actually distress me a lot. When they finally listen it's ok, but that's the reason of the labels isn't it - for them to understand in a nutshell what I'm going through.
Personally it's also validation of what I experience and that I struggle with those things. It also helps me to understand them - kind of like when I have a panic attack but don't realise it until several hours or days later when I recognise my behaviours were nervous and agitated. I'm usually so dissociated from my sense of self that I don't really experience the emotions, I somehow have them with out knowing, like another part of me is having them - so when the issues that the other part of me has, diagnostically confirmed it's a relief and means I can start to understand it and make progress.
Lastly and I believe this to be the biggest thing and this was a recent realisation: The fact that it's diagnosed means it wasn't my fault. I could nearly cry at the relief this gives me. It's not my fault, sure that doesn't mean I can use it to hurt others, but it's as close as validation comes to proving what happened is real and I'm not making it up and that I'm not incapable because I'm not good enough, but I've been through a lot and need to recover and heal and learn and grow.
Not having it has hindered me and retrospectively the misdiagnosis also hindered me. Having the correct diagnosis has only helped me so far.
Any way sorry for the long message, but I think that about covers it!