Well I could tell you my experience, but this also can include why it's helpful to get a diagnosis for your child (whatever that condition is).
In all honesty though, reading your question, I had to think long and hard why having your child diagnosed is an advantage. I must say, if my child was diagnosed and I wasn't being a proactive parent about what his needs, I believe my DS would of been left dead in the system, or it would of been too late to get the need he requires.
That's one thing, in my borough, a diagnosis doesn't really mean anything, it's about what your child NEEDS. If I didn't complain and shout at the professionals that my DS needs help, they wouldn't of done anything with him.
If you want your DS to go to a specialised special school or specialist unit, you need a diagnosis, otherwise your child will end up at a special school that takes all needs of children.
Once you have an autism diagnosis, they would most very likely not take into account of any other or possible conditions your DS may have. Everything is blamed on the autism. For example, I suspected DS must of had something else going on besides the autism, in his case, the autism didn't really explain everything. The professionals also suspect this too, but instead they gave him an additional diagnosis of Moderate learning difficulties. I thought DS may have had something called Verbal dyspraxia, but the Autism team didn't want to hear of it. So I went to a private speech and language therapist who actually assessed DS and concluded that he did have verbal dyspraxia and that you can have Autism and verbal dyspraxia.
Due to me appealing his statement, having saw the various of independent professionals, the specialised SALT who assessed DS for tribunal reasons, concluded that he indeed does have verbal dyspraxia as well as oral motor dyspraxia. DS has been recently seen by the NHS Nuffield speech centre and the team said when assessing DS for 5 mins, they already knew he had the dyspraxia condition and the head of the centre also told me that you can indeed have dyspraxia as well as ASD. Oh and funny enough, when DS was assessed by an educational psychologist, he concluded that DS most definitely did not have moderate learning difficulties and came out as high average cognitive ability. So there is a danger of misdiagnosis of very young children.
I must say, with a diagnosis you are referred to a various of sources, that you would not get if you didn't have a diagnosis. eg. Autism outreach team etc.