I know people who did it for religious reasons and I know people who did it because their kids needed extras to be able to attend school at all, and regular schools didn't have the staff and/or the ability, ie: one friend has an adopted son with FAS who, when he was stressed, needed to run outside around the building a few times before he could sit still for a few more hours.

He was finally able to graduate at 21 ... a crowning achievement for him and his family.