How can giving back help someone with a disability?

Great question. Volunteerism, giving back, community service, and sponsoring another in a peer to peer assistance format, are champions for enhancing your personal growth, spiritual enrichment, and happiness. Joy and pleasure often come from involvement with others in a one on one level of service. Just because you have a disability, doesn't mean you don't have great worth in the lives of others around you.
Distraction. All chronic disease, pain and depression is always better if you are not focused on it. Helping others in need is a great way to put yourself in perspective and more importantly distract you from your own pain. Be sure to find something to do that you can engage safely in and not put yourself or someone else at risk becasue you may be distracted by your own pain or disability.