It prevents burnout and eases the pressure.
If you know that you can go to the sensory room to escape if needed, that's less pressure than if there is no escape and end up losing control or ending up in skiving the toilets and facing the discipline system.
Diagnosis helps his teachers read him. They understand that he's autistic and not being rude when he doesn't look at them, curls up on his chair (because standard sitting is uncomfortable for his hypermobile, dyspraxic limbs) or when he answers with a slightly "off" tone or word choice or has odd points of view that are logical to him that he struggles to articulate and explain.
He did social skills work in y7 when he didn't make friends after transition (did not transfer with friends) and met some other people on the same wavelength. He doesn't like the word "friends" but he finds them to be reasonable company and they also find the concept of meeting up outside school weird so there's no offence about 6 weeks of no contact each summer.
He can do his exams where there are fewer sensory distractions. His dyslexia and dyspraxia diagnoses allow him extra time to process language and express his ideas so he can demonstrate his knowlege in time.
We used to get regular meltdowns lasting up to 4 hours which was the crisis point that lead to assessment at age 7 (diagnosis at 9). With age he shuts down more. I've learned to pace him better; that the first two hours after school every day are a write-off. I don't fight his insomnia with arbitary bedtimes (although we could now do with a melotonin prescription that needs organising, but at least that is on his medical records)
The little bits of understanding at home and at school and adjustment to routines reduces the crisis points, and helps us manage them better when they do arise.
We're lucky that there have been few issues in school, but when they have arisen, pastoral conversations between home/ school/ him have de-escalated and not provoked his strong sense of justice that could antagonise issues. When he had a sibling dispute exacerbated by his autistic difficulties of struggling with change when his sibling moved into "his" school, treating it pastorally was far more effective than the standard behaviour system that would otherwise be used for that behaviour. He also can very rarely say "sorry"; that doesn't mean that he doesn't recognise that his behaviour hasn't met standards and a better approach would be to do x, but his feelings of justice about the trigger override being able to use that particular word. What he does say is sincere, and that's more important long term than parroting a specific word, but society likes "sorry".
He hasn't required many formal interventions but that doesn't mean that his needs aren't real and don't deserve formal recognition. When something more significant occurs, it can be responded to appropriately and faster and he isn't 1-4 years away from that recognition.
Diagnosis has enhanced his quality of life because he is understood better.
I had friends with similar profiles who went undiagnosed through their teenage years in the 90s, and that lack of self and societal understanding has caused more serious crisis points until late diagnosis in their 40s when their world finally made sense.