Social Security defines by regulation the "acceptable medical sources" that can establish your medically determinable impairments and their severity. Physicians and psychologists are of course included. So are optometrists (only for matters of visual acuity and visual fields) and podiatrists (only for impairments of the foot).
Notably, licensed clinical social workers, counselors and therapists are not included. This can pose a problem for the disability lawyer developing the claim.
The purpose of a medical source statement is to establish your work-related limitations. Your claim is stronger if the statement comes from an acceptable medical source, because it is more persuasive.
However, sometimes a statement from a "non-acceptable" medical source is all you are going to get, because that provider is the only person the client has been seeing for medical care. Social Security Ruling 06-03p states that an opinion from a non-acceptable source may be persuasive evidence of the severity of your medical condition, and the limitations imposed by your condition. However, a non-acceptable source cannot establish a medically-determinable impairment.