In "The Adventure of the Speckled Band," why does Helen Stoner come to see Holmes?

I actually don't think that Helen Stoner was aware that she was in grave danger when she went to visit Holmes. She went simply due to the low whistle which she heard in the middle of the night she had to spend in her sister's room.

It is Holmes who suspects that there is a danger to her life and makes an appointment to visit her soon so that he may investigate the cause behind the whistle she heard. Holmes makes a plan to discover the reality that night and gives strict instructions to Ms. Stoner. She is to return to her old bedroom while Holmes and Watson enter the room in which she heard the whistle and wait.

Eventually, the whistle sounds and a snake enters the room through an air ventilator from the father's room next door with the use of a rope that had been placed on top of the bed. Holmes hits the snake which in fear returns into the ventilator and ends up biting the father. The snake is revealed to be the most poisonous snake from India.

Although Helen Stoner came with the feeling that there was something strange taking place in her home, it was really Holmes who understood the seriousness of the matter and the risk to her life thanks to his keen sense of reasoning and intuition.