The functional theory how the eye focuses assumes that all the muscles in and around the eyes are involved in focusing the eye, including the famous ciliary muscle. Of course, the brain has to understand what a person is seeing.
When you walk you do not use only one muscle - right?
You use several muscles as well as balance and motion when you walk. The brain sends signals to the various muscles telling them when to contract, by how much and for how long and when to relax. Not only can you walk, you can also run and dance.
Optometrists and eye doctors practice what is called evidence-based medicine. This means that procedures are tested and proven scientifically. Optometry is applied optics, the tests and treatments are only to ascertain and treat the symptoms.