"“Do you know, Watson,” said Holmes as we sat together in the gathering darkness, “I have really some scruples as to taking you to-night. There is a distinct element of danger.”..."See in text(The Adventure of the Speckled Band)
The reappearance of Dr. Roylott at Stoke Moran reminds the readers of the danger he represents. The author brings Rollout back into the story to heighten the tension of the investigation.
"“You have been cruelly used,” said Holmes...."See in text(The Adventure of the Speckled Band)
This is shows that Dr. Roylott is a menace to his stepdaughter Helen, adding to Roylott's violent characterization and emphasizing that Helen is in urgent need of protection, which only Holmes and Watson can provide.
"Your presence might be invaluable..."See in text(The Adventure of the Speckled Band)
Watson's presence is necessary for the possibility that there might be an encounter with Dr. Roylott, who would be quite capable of trying to harm both of them if he found them inside his home. Watson is carrying a revolver at the request of Holmes, and Roylott, like any typical country resident of the times, would surely have means of protection.
"Five little livid spots, the marks of four fingers and a thumb, were printed upon the white wrist..."See in text(The Adventure of the Speckled Band)
Recall earlier in the story when Sherlock Holmes leaned forward and patted Helen sympathetically on the forearm. He obviously noticed that she was trying to conceal something with the sleeve of her dress, and he wanted to get a closer look. Here is physical proof that Dr. Roylott is an exceptionally powerful and dangerous man. This increases the tension and conflict between Holmes and Roylott.
