A-One's RFC is a way to measure a person's ability to perform various tasks that all or most jobs will require. If you have one RFC performed (the cost is around $850 for this 3-4 hours testing) this test gives great weight to your functional ability. That is what the Social Security Administration is all about. The government wants to measure your ability to work. You can testify about your ability to perform or not perform a job but that is not enough. They want highly objective evidence. A good quality 15 page report with all types of measurements of your functional abilities, to stand, sit, walk, stoop, lift, twist, climb, bend, etc., is then given to your primary doctor who will review it for accuracy and if the doctor feels it is accurate then he/she will sign off for the authenticity of this document. That type of information is what Social Security Administration needs in order to determine your eligibility for benefits.
Obviously they need more documentation than just the RFC. They need all kinds of medical records that coincide with your treatment and demonstrate your lack of ability. But the RFC goes a long way in establishing your functional abilities which gives the Administration at Level III the Judge an opportunity to help you win your case.