In "The Adventure of the Speckled Band," why were Sherlock Holmes and Dr. Watson at the Crown Inn?

After Sherlock Holmes has finished examining Helen Stoner's and Dr. Roylott's adjoining bedrooms, Holmes decides that he and Watson must spend the night in Helen's room after her stepfather has gone to sleep. She is to spend the night in her old bedroom and leave a light in the window of the room adjacent to Dr. Roylott's after he has retired for the night. Holmes' reason for renting a room at the nearby Crown Inn is shown in this exchange of dialogue:

“In the first place, both my friend and I must spend the night in your room.”

Both Miss Stoner and I gazed at him in astonishment.

“Yes, it must be so. Let me explain. I believe that that is the village inn over there?”

“Yes, that is the Crown.”

“Very good. Your windows would be visible from there?”

“Certainly.”

Watson goes on to explain:

Sherlock Holmes and I had no difficulty in engaging a bedroom and sitting-room at the Crown Inn. They were on the upper floor, and from our window we could command a view of the avenue gate, and of the inhabited wing of Stoke Moran Manor House.

The two men do not see a light in Helen Stoner's window until about eleven o'clock that night. Dr. Roylott has finally gone to sleep. They walk along the dark road to Stoke Moran and then enter the grounds through a gap in the crumbling brick wall. Helen has left the window open and retired to her old bedroom. Holmes and Watson climb through the window and close the shutters to make the room completely dark. They have to sit in the darkness in utter silence from a little after eleven until almost three-thirty in the morning. Dr. Roylott is equally silent on the other side of the wall, but they can smell burning oil and heated metal from his dark lantern.

Then suddenly another sound became audible—a very gentle, soothing sound, like that of a small jet of steam escaping continually from a kettle. The instant that we heard it, Holmes sprang from the bed, struck a match, and lashed furiously with his cane at the bell-pull.

Up to that point Holmes has not told Watson what they are waiting for. The reader is also kept in suspense until he realizes that the so-called "speckled band" is a snake and that it must have been on the bed for some little time while they were waiting in silence in the dark. Dr. Roylott blew the whistle to call the trained snake back up the bell-rope and through the ventilator so there would be no chance of Helen waking up early and finding herself in bed with a snake! He had only done this with Helen on one night, but he must have sent the snake into the room for several nights before it finally bit and killed Helen's sister Julia two years ago. Helen panicked the first time she heard that whistle at around three o'clock in the morning because her sister had told her about it shortly before her mysterious death. That was why Helen came to London immediately that same morning to ask Sherlock Holmes for his advice.