I just re-read OP's original post. Particularly this:

How different would my childhood have been if I had been evaluated when I was younger?

There are many times in the sessions with my new therapist where we have talked about things that have happened in my childhood that were very difficult for me to deal with, and that were sometimes traumatizing. I desparately wish that I would have been diagnosed at a young age and had the resources that many autistic kids have these days. I am convinced that having a childhood autism diagnosis would have helped me to better handle situations that I have experienced which have left deep wounds in my psyche.

tl;dr: My formalized autism diagnosis has been very valuable to me because it has guided me to an autism-specialized psychotherapist, who has then provided me with a wealth of understanding and therapeutic techniques that center around autistic behavioral traits.

I’m a 54yo male who was diagnosed with autism (mild) last year. I (and many people I know) have suspected that for a while, but I wanted to find out exactly where I stand on the spectrum. For my interests and needs, the diagnosis was very much worth it. I have gotten a lot of value out of it.

I went to a psychologist who specializes in autism testing for all age groups. Autism testing is all that she does, so she doesn’t do ongoing therapy with any clients. The diagnosis process involved a 4-hour session with an interview portion, and then a number of different computer-based tests (I believe there were 5). After reviewing all of that, the psychologist wrote up a 13-page document that had the details of my test results, a section on the types of behavior traits that I have which are likely due to my autism, guidelines for continued autism-centric psychotherapy, and even guidelines for non-autistic people on how to communicate with someone with autism. It is a very thorough writeup.

As someone else mentioned in this thread, I also wanted to do this for closure. I wanted to know for sure where I stand on the spectrum, and I got a lot of information about that. By learning the language that is used to describe autistic behavior traits, I have found that it is now a lot easier for me to communicate with others about how autism affects me.

The biggest benefit from being diagnosed with autism is that I was able to find a psychotherapist who specializes in doing cognitive behavior therapy with autistic people. Working with her has been very eye-opening. I have worked with about half a dozen therapists over the past 25 years, but I never felt like any of them ‘got me,’ and I never felt like I benefitted much from their therapy sessions.

With my new autism-centric therapist, I finally feel like I can talk to a professional who really understands what it’s like to be me, which is quite refreshing. This is important because I have learned that people with autism will often respond better to certain types of cognitive therapy, and worse with other types. Having a good diagnosis can help a therapist hone in on appropriate types of therapy that are more effective with autistic people.

She has helped me immensely with understanding things like how I respond to trauma situations (which can be amplified in an autistic person). She helps to bring clarity to my internal thought processes when I deal with stressful problems in my personal life. She regularly talks about scientific studies that show correlations between autism and other health-related issues, like nutrition and medication sensitivities. Apparently, due to the genetic makeup of many autistic people, there are certain medications that will perform differently in an autistic person than someone who is neurotypical. This is especially true with medications that affect neurotransmitters, like antidepressants or anti-anxiety meds.

Everyone is different, and any individual would have to weigh the pros and cons of getting a formalized autism diagnosis. I just wanted to offer up what I believe has been a very positive experience with getting diagnosed as an adult.