One of the important benefits of Social Security Disability is that it also provides you with access to medical coverage.
Once Social Security determines you are disabled, approximately 24 months after your initial payment date you are eligible for Medicare coverage. Onset date simply means the date Social Security says you became disabled. In some states, depending on the amount of your benefits, the state will pay your Medicare premium for you.
While you are waiting for your Medicare to kick in, the State of Illinois will cover your medical needs through Medicaid.
This is another reason that Social Security Disability is so important for people with serious health conditions.