So essentially all autistic people share difficulties in the same areas of development, however how much and in what way they are impacted is very individual.
As for why a diagnosis is essential for understanding. Imagine everyone around you were all laughing at this amazing joke, they all got it and thought it was hysterical. Everyone is sharing in this joint event.
But you just don’t get it.
You hear the joke and understand it’s meant to be funny and you’re supposed to laugh but you just don’t get it - at all.
So, for years you play along. Pretending you totally get it and learn to laugh at the right time. All the while thinking that you’re stupid for understanding something so simple that literally everyone else gets instantly. For years you put yourself down.
Then suddenly you find out that actually, the joke. It’s in a different language!!!! That’s why you didn’t understand it. You’re not stupid, there’s not something wrong with you, you just speak a different language than the one that the joke is told in. Not only that, but there are others who don’t speak that language too.
The relief. The knowledge that you belong to a whole community who don’t get the joke too. Who wouldn’t want that?
(My apologies to any autistic people if my analogy is clumsy or causes offence, it’s unintentional but please don’t hesitate to let me know)