Helen Stoner did not have much money of her own. Her stepfather controlled her assets and probably only gave her a small allowance. She had very little freedom, like most unmarried women in Victorian times. She could not have gone to the police for help because no crime had been committed, except of course for the murder of Julia Stoner two years before, which was a closed case. Helen could not go to another private detective because there probably were no private detectives in those days, and anyway she couldn't afford to pay them. Sherlock Holmes is compassionate enough to help a person like Helen without asking for a fee. He is also intrigued by this so-called "Locked-room mystery." If Helen hadn't heard of him, her plight would have been dire. The author, Arthur Conan Doyle, takes pains to show how she knew about Holmes and also knew he would help people in distress in some cases.
I have no one to turn to—none, save only one, who cares for me, and he, poor fellow, can be of little aid. I have heard of you, Mr. Holmes; I have heard of you from Mrs. Farintosh, whom you helped in the hour of her sore need. It was from her that I had your address. Oh, sir, do you not think that you could help me, too, and at least throw a little light through the dense darkness which surrounds me? At present it is out of my power to reward you for your services, but in a month or six weeks I shall be married, with the control of my own income, and then at least you shall not find me ungrateful.”
Helen is engaged to a Percy Armitage, but she does not feel that he can be of much help to her. He has no experience as a detective, and he obviously could not come and spend a night or two in her bedroom. Therefore the chances are that she would die in the same horrible manner as her sister. Dr. Roylott cannot afford to give her the money she would inherit upon her marriage. He would do his utmost to have her killed by his snake. She might continue to hear the whistle, but she would have no idea that an Indian swamp adder was slithering through the ventilator, down the dummy bell-rope, and onto her bed every night. The snake comes from a hot, humid climate. It would not try to escape from the house, especially since the author has emphasized that the weather is unusually cold. Instead, the snake would be likely to crawl right under the covers and curl up beside the sleeping Helen. It probably wouldn't bite her unless she turned over in her sleep and was lying right on top of it. But this was inevitable. Sherlock Holmes saved her life. Otherwise she would have been dead within a short time.
Dr. Grimesby Roylott would undoubtedly come under suspicion, since both his stepdaughters would have died the same way. But he would have disposed of the snake after it was no longer of further use to him, and it would have been difficult for Scotland Yard to make much of a case against him. That explains why he is so concerned when he finds that Helen has come to London to consult Sherlock Holmes and why he tries to scare Holmes away from Stoke Moran. Holmes, as Roylott knows, could be a lot more dangerous than any Scotland Yard detective.