After having watched the movie, the question returns, only now it is answered.
Throughout the film, Mrs. Travers has numerous flashbacks of her childhood and namely of her father. As the story progresses, the spectator learns that her father, though very loving, struggled with staying employed and also had an alcohol problem. The bond between father and daughter was emotionally moving for me. Mrs. Travers was reliving her past showing how her father's circumstances went from bad to worse--how all she wanted to do was help him, save him.
And that's the clincher.
Mary Poppins didn't come to save Jane and Michael but to save Mr. Banks. In doing so, Mary Poppins was also saving Mrs. Travers father.