Why do I need a teacher’s assistant for only part of the day for a diagnosed severe disability child or special needs child?

So I’ve been a primary teacher for 12 years. I had a similar question as this last year as well. But for me it’s happening more and more. I’ll get a child placed in my class that has a severe disability. An IEP will be written. But I will only get support for that child for a partial day. I had one where I had a para only for 45 minutes a day to help with this child. Do they think this child just goes to sleep the rest of the day? Does it matter if it isn’t the 504 or IEP. I am just confused as to how I am supposed to provide the best learning for this child if they are truly in need of that extra support. I’m not even trying to be lazy, I would just assume you would need more help for a child that needs one on one help.