Note, I am not a doctor, and this is just my current understanding:
To make it easier to get insurance to pay for support. That's a big chunk of it at least. If each of these profiles was a separate diagnosis, it'd be way more common for insurance to deny paying even things like therapy for stuff in the long tail. Rather than have to re-argue again and again that people are worthy of support, the diagnosis broadens to help get that support to all that need it.
This is also true for ADHD, and I'm sure other diagnoses as well.
There's also a layer of making it easier to get kids the academic support they need when that support requires meeting certain officially recognized criteria. "Oh, little Jimmy's psychologist says he has Cognitive Disengagement Syndrome? We don't consider that as needing accommodations."
It's already sometimes exceedingly challenging for parents to get schools to actually support their children when it's a commonly understood and accepted diagnoses like ADHD.
To make it easier to get insurance to pay for support. That's a big chunk of it at least. If each of these profiles was a separate diagnosis, it'd be way more common for insurance to deny paying even things like therapy for stuff in the long tail. Rather than have to re-argue again and again that people are worthy of support, the diagnosis broadens to help get that support to all that need it.
This is also true for ADHD, and I'm sure other diagnoses as well.
There's also a layer of making it easier to get kids the academic support they need when that support requires meeting certain officially recognized criteria. "Oh, little Jimmy's psychologist says he has Cognitive Disengagement Syndrome? We don't consider that as needing accommodations." It's already sometimes exceedingly challenging for parents to get schools to actually support their children when it's a commonly understood and accepted diagnoses like ADHD.