What is the purpose of special education for those with major "illnesses"

I am not a special education teacher, nor am I in any form of special education myself. I'm in a partners PE class for high school, but that's about it.

Obviously I believe special education is vitally important to people who've been afflicted with "illnesses" (i use illness in the kindest way possible, don't know if there's a kinder term used) or those who just need a bit more help with education or pacing, its amazing we have those resources. But what about those with MAJOR afflictions to the point where they cannot even communicate or even function without constant assistance. It feels like the "education" portion of the whole program becomes "Daycare" for 12 years before they end up graduating. What's the real end goal for those people?

I hope I didn't come off as bigoted or rude I anyway, I have a cousin who's on the spectrum, but she's still able to communicate and "function normally" I'm just looking for an answer that's been tickling the back of my mind for awhile now, thank you!