Why do you think it is the schools that are doing it?
My daughter has severe dyslexia and is ADHD.
The dyslexia was diagnosed when she was 7.5 years old - and was incredibly obvious given she was unable to read, or spell but was very advanced verbally with wide vocabulary and obviously clever. The discrepancy was huge and Primary paid for all the assessments and Ed Psych reports.
Why is she dyslexic? It's very heritable. DH is mildly dyslexic, one of his brothers is severely dyslexic, my brother and mother were mildly dyslexic.
ADHD is highly heritable - around 80% genetic. Two out of 4 siblings with it, and 5/8 cousins. I knew DD was ADHD since she was about 5, but didn't get her diagnosed until Y9 for various reasons. Medication has been life-changing for her.
In terms of benefits - it helps her understand why some things are a major struggle. She has access arrangements for exams, lots of tech to help, and medication to help manage the ADHD.
It's not simple to tick the boxes, and giving your child stimulants every day is absolutely not something you would ever want to do without very good reason and having tried everything else.
Oh and there is no money involved unless you count the vast amounts that I've ended up spending on tutors, tech, whacky pens and overlays and helping my child excel at the specific area where she happens to be unusually gifted.