Why does Helen Stoner come to see Holmes?

Helen Stoner comes to see Holmes because she wants him to investigate her sister's death and also because she fears that she will meet the same fate as her sister. Essentially, Helen wants Holmes to figure out how her sister really died. She suspects that her stepfather was responsible for Julia's demise, and she fears that she may soon become his victim.
In the story, Helen and Julia are twin sisters. Julia died two years ago, and at the time of her death, the county coroner could not find any "satisfactory cause of death." Prior to her death, Julia had asked Helen about a strange, low whistle she heard over the course of several nights. For her part, Helen was perplexed, as she herself had never heard anyone whistling in the night.
Helen reports that Julia left for bed after inquiring about the whistling sounds. Later that night, Julia emerged from her room pale-faced. She appeared to be in great pain and was convulsing terribly. Julia's only coherent words were about a "speckled band." Helen does not know the meaning of Julia's words, and that is why she has come to see Holmes.
Helen is also concerned because she is soon to marry Percy Armitage, her fiance, and she has since heard the strange whistling sounds Julia mentioned. Helen tells Holmes that she heard the sounds while sleeping in her dead sister's bed.
Helen comes to see Holmes because she wants him to investigate the cause of Julia's death. Additionally, her fears about meeting the same fate as her sister are also a motivating factor for her visit.

Helen Stoner comes to see Sherlock Holmes at his 221 B Baker Street residence on the recommendation of Mrs. Farintosh, a woman Holmes had helped with a matter regarding "an opal tiara."
Miss Stoner is plagued by vague fears and suspicions that others, including her fiance, do not take seriously. She feels that she is being dismissed as a nervous woman. Miss Stoner believes that Holmes "can see deeply into the manifold wickedness of the human heart" and help her.  Her mother is dead, and her stepfather treats her badly; as a result, she has no one else to confide in or help her. Her twin sister died under mysterious circumstances after commenting on hearing a low whistle in the night.  On the night before Miss Stoner arrives at Holmes's apartment, she heard the whistle as well. Moreover, she does not understand her sister's dying words: "It was the band! The speckled band!" Miss Stoner wants Holmes to find out what caused her sister's death and how the whistle and the speckled band figure in the case.
https://sherlock-holm.es/stories/pdf/a4/1-sided/spec.pdf