DS got his official diagnosis of autism a week after he turned 3. (He is now 7.) Here is how getting an official diagnosis benefited us:
In my state, coverage for autism therapies is not mandated, so some insurance companies cover it and some do not. At the time we got the diagnosis, our insurance did cover speech and OT for autism spectrum disorders. Previously, DS had received this services with a diagnosis of developmental delay/language delay and they had NOT been covered under those diagnosis. Currently, our insurance does not cover services for ASD, (new employer, new insurance), but I'm glad that for the first couple of years we had insurance that did.
Explanation for issues:
The diagnostic process took nearly 5 months and was very thorough. Tests were done to rule out things like metabolic disorders, known genetic disorders, seizures, hearing problems, etc . After everything else that could be possible was ruled out, autism was what was left. So we were able to feel confident that we had a correct diagnosis and we were not missing some other medical issue that should be treated first. (DS's severe vision problems were discovered at this time.)
Prior to getting the official diagnosis, we had family members who blamed us and our parenting style for all of DS's behavioral difficulties and language problems. So in a way, getting the diagnosis was a relief because it reassured us that wasn't true. Then we were able tell those relatives that a whole team of doctors at Children's Hospital said that it wasn't our fault, and that DS had a disorder. We were able to start educating those family members about what was really going on.
Getting a diagnosis made it possible for us to find information about various treatments and techniques we could use for helping make like easier at home, at school, and in the community.
Soon after DS was diagnosed, I found something really great in one of the books I was reading (I read so many I have no idea which one it was, but this story always stuck with me.) A Dad was explaining why he sought a diagnosis for his son, while friends and relatives urged to fight against a "label" and let his son be "just Jake". The Dad told them: " There is no treatment for services for 'just Jake'. Nobody is doing research on 'just Jake'. No one has studied the long-term development and prognosis for 'just Jake'. Autism doesn't make him less Jake, but it gives us these things." That really rings true for me.
Resources & services:
We found that having an autism diagnosis opens the door to many resources and services that are not available with a "developmental delay" or a "language delay" diagnosis. This has been especially true in getting services from the school.
Getting a diagnosis led to us becoming active in the local autism community. When DS was a preschooler, we went to a weekly autism playgroup and we go to support group meetings, autism events, etc. Our local autism community is wonderful and being a part of it has benefited us so much. DH and I got to meet other families who have kids with autism and exchange information about doctors, therapists, schools, interventions, etc. We have swapped IEPs with other families so we could learn what to ask for and see what services other districts offer. DH found it extremely helpful to talk with other Dads and to see how they interact with their kids. DS loves being around other kids with autism and it is good for him to know a group of kids who are like him.
So that is what we get out of having a diagnosis. I hope that helps.