...if it tells you nothing at all about the person with autism?
A diagnosis of autism tells you that someone has a congenital neurological difference. It tells that person that the reason they're struggling is because their wiring is different to others', not because they are stupid or socially inept. It is important that this is recognised, as it is a lifelong difference.
It tells their teachers that the reason for their screaming, not following instructions, or other 'bad' behaviour is not naughtiness. It tells me and my daughter that her OCD is lifelong, not a problem that can disappear with therapy.
The support needed by any individual should be determined by the attributes whether they are diagnosed with something or not
Two young children may both be crying in assembly and reluctant to go to school generally, but the support needed could be very different: one has has a difficult start to life and is shy; the other is autistic. The help that is needed for each will be different and may involve different agencies. The first may get better within a short while with a little understanding and increasing maturity; the second's difficulties will be lifelong and require different accommodations. It's not just the symptoms, it's the cause of the symptoms that's important, which is how a diagnosis helps.
It would be great if there were 4 or 5 different types of autism, easily identifiable and diagnosible, with symptoms that are easy to anticipate and mitigate so that adjustments could easily be put in place, but it is far more complex and varied than that, unfortunately.