If you have your health, you have everything - or so they say. Those who are suffering with an illness or injury that greatly impacts their health and their quality of life would tend to agree with this statement. Being ill or injured is about more than a poorer quality of health. It is about not being able to do the things you want to do, because of poor health.
This would include being unable to work or earn a living wage due to illness or injury. There are SSD benefits available to those who apply and are qualified to receive these benefits. For those with illness, there is a long list that includes illnesses that are commonly or regularly approved for SSD benefits per the Social Security Administration. However, not all illnesses are listed.
So where does that leave ill people with rare or unheard of illnesses or conditions? It doesn't leave them completely out of the loop. For those with an uncommon illness who are seeking SSI or SSDI, the medical condition must be a medically determinable impairment. Once that has been established, the medical condition must limit your residual functional capacity or RFC. RFC is measured by exertion levels and also non-exertional activities when one seeks Social Security Disability Benefits for Illness.
Essentially, if a medical condition exists that is not on the SSA's approved list of conditions, there are more requisites to meet. Meeting these requisites will get an injured or ill person one step closer to receiving SSD benefits. These benefits can be the difference between having enough to live on or not. So it is clearly important to the injured or ill to receive these benefits.
Source: socialsecurity.findlaw.com, "Medical Conditions that Qualify You For Disability claims," Accessed Dec 11, 2017