I will try to make my answer as simple as possible;
1. Any movement is made by our body using muscles and nerves.
2. Muscles are of three types,
2.a Voluntary Muscles are those which we can command to move. These muscles move by the command given by our brain, which is relayed to the muscles, which carry out the desired movement.
2.b Voluntary muscles are called as Skeletal Muscles, since they are attached close to the bones and obviously use the bones in our body to pull or push.
2. c The other two types of muscles are 'involuntary muscles', meaning these muscles are controlled by the central nervous system (CNS).
2.d The two types of Involuntary muscles are
(i) Smooth muscles: Like in our intestines, stomach lining blood vessels etc, Their movement we are not much aware off.
(ii) Cardiac muscles.: Are basically muscles which are found in the heart.
We are however concerned with Voluntary muscles
3. Voluntary muscles, work in pairs; When one contracts the opposite set relaxes. For example flexing your biceps.
4. When we trace the 'S' of the fig. 8, all the necessary muscles, ENCASING OUR bones from the shoulder joint, elbow, wrist (major ones) are exercised alternatively in ONE PLANE ONLY.
5. When we reverse direction to complete the fig. 8, all the necessary muscles are exercise in pairs in the OPPOSITE PLANE.
6. Besides these major muscles, there are other muscles which perform supporting role, these are all the other muscles in our body which support the body and hold it rigid, while the fig. 8 exercise is on.
7.FINALLY: The ultimate objective is to hold the SA as steady as possible within the Aiming area That is why, ALL THE MUSCLES ENCASING the shoulder to hand, MUST BE EXERCISED. A complete exercise to ALL the muscle pairs can be done best through the fig.8.
Sincerely hope that I have been lucid.
Best Regards and Oh! Lovely question after a long long time.