After Helen Stoner's long interview with Sherlock Holmes, her stepfather Dr. Grimesby Roylott bursts into Holmes and Watson's sitting room and shows himself to be a powerful and dangerous man. He tells Holmes:
“I will go when I have said my say. Don't you dare to meddle with my affairs. I know that Miss Stoner has been here. I traced her! I am a dangerous man to fall foul of! See here.” He stepped swiftly forward, seized the poker, and bent it into a curve with his huge brown hands.
Holmes fully intends to go to Stoke Moran to inspect the premises in person, and Dr. Watson volunteers to accompany him. As they are preparing to leave for the train station, Holmes tells Watson:
I should be very much obliged if you would slip your revolver into your pocket. An Eley's No. 2 is an excellent argument with gentlemen who can twist steel pokers into knots.
This strongly suggests the possibility that they might run into Dr. Roylott inside his home. In that case Roylott could be very dangerous. No doubt he would feel justified in shooting both intruders. A man like Roylott would certainly have more than one gun available. His most likely weapon would be a shotgun.
The author, Sir Arthur Conan Doyle, wanted to make the inspection of the rooms at Stoke Moran dramatic by adding the element of danger to the visit. Fortunately, Dr. Roylott stays in London during the time that Holmes and Watson travel to Stoke Moran. But there is an atmosphere of danger during the whole time that Holmes is carefully inspecting the exterior and interior of the decaying mansion. They never see Roylott alive again. The only time they see him after his stormy visit to Baker Street is after he has been bitten by his own snake.
Beside this table, on the wooden chair, sat Dr. Grimesby Roylott clad in a long grey dressing-gown, his bare ankles protruding beneath, and his feet thrust into red heelless Turkish slippers. Across his lap lay the short stock with the long lash which we had noticed during the day. His chin was cocked upward and his eyes were fixed in a dreadful, rigid stare at the corner of the ceiling. Round his brow he had a peculiar yellow band, with brownish speckles, which seemed to be bound tightly round his head.
Sherlock Holmes will occasionally ask Dr. Watson to bring along his pistol in other stories when they are going on a mission that could be dangerous. However, Conan Doyle always leaves the physical action to his hero Sherlock Holmes, and Watson never has occasion to fire his gun. For example, at Holmes' request Watson brings his pistol to the bank storeroom in "The Adventure of the Red-Headed League" because they will be dealing with John Clay, a very dangerous criminal. But it is noteworthy that it is Holmes who disarms Clay with his hunting crop when Clay tries to use his pistol. In "The Adventure of the Speckled Band," it is Holmes who beats the snake with his cane and causes it to retreat back up the bell-rope and through the ventilator.
The fact that Homes wants Watson to bring a revolver shows that he was wary of possible dangers.