Although Helen was deaf and dumb, she was extremely intelligent. She had learned how to manipulate her parents and they could not handle her, so they indulged her. Annie knew this. Annie felt that Helen would have to totally depend on her for everything -- food, water, attention -- if she was going to be able to make any headway with the strong-willed and out-of-control young girl.
Helen's father did not want to agree to let Annie have full control of Helen. He wanted to fire Annie. Mrs. Keller finally convinced him to go along with it. She loved her daughter and in the play, she seemed to sense that there was something unique about Annie and that this young woman would finally be able to help Helen when she and her husband could not.
In order for Annie to really teach Helen language, she needed some cooperation from Helen. To get said cooperation, she decided Helen needed to be COMPLETELY dependent on HER so that she would have many opportunities to feed language into Helen and teach her manners.