Why was it necessery for helen to be completley dependent on annie during her learning?

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.