When applying for social security, VA benefits, or some other disability benefit, the adjudicator reviewing an application wants to see not only medical evidence of a diagnosis or ability, but they also want to review documentation of treatment history, outcomes of such treatments, and current functional status or abilities. If the disability is physical in nature (musculoskeletal; involving joints or muscles), the adjudicator may want to review records of physical and/or occupational therapy treatments and progress notes/evaluations from those clinicians.
It is important to have detailed and accurate records of treatment regarding any treatment received in the past, and any treatment currently being received. It also helps to include clinical reports that document current functional abilities and status.
Some agencies, like Social Security Administration, request a medical source statement explaining what functional abilities remain despite the disability or impairments. These need to come from a licensed medical professional. While it is best to get this statement from your medical doctor, documentation from physical and occupational therapy (or other medical professionals) that are not involved in your case or treatment help and can be considered by those agencies.