Sherlock Holmes calls himself a "consulting detective," but actually he is what we nowadays call a "private detective," not unlike Dashiell Hammett's Sam Spade or Raymond Chandler's Philip Marlowe. A private detective works for money. Only wealthy people can afford him. The others have to go to the regular police, who may or may not be willing to assist them. But Sir Arthur Conan Doyle realized that he couldn't keep writing Sherlock Holmes stories in which an aristocrat hires Holmes on a private and usually delicate matter. Doyle wanted to give his hero motivations that would bring him into contact with the entire spectrum of English people. This meant that Holmes would have to take on cases pro bono. Holmes works for some of his clients for nothing because of various reasons. One is to assist the police. This is helpful to Holmes indirectly because the police are grateful for his help and for giving them the credit for solving the crime, and they are therefore happy to assist him whenever he needs it. Holmes is only able to get involved in murder investigations because of his cordial relations with the police; otherwise such cases would be strictly off limits to private or amateur detectives. Another reason, or motive, is curiosity. Holmes frequently complains that his brain needs problems to solve, so he takes on petty problems because they intrigue him. Watson mentions that Holmes uses strong drugs when he has nothing to occupy his powerful brain; so he is better off working for nothing than indulging in cocaine or morphine. Holmes also helps ladies in distress, as he does in "The Copper Beeches" and "The Adventure of the Speckled Band," for example. Holmes is indifferent to romantic love, but he has a strong sense of chivalry. He also is patriotic and has a strong sense of justice and the importance of preserving law and order. He is a champion of law and order and of British justice, as he shows in his ongoing conflict with Professor Moriarty. He takes on non-paying cases that will help his country. He also takes on cases in which he senses that an innocent man is going to be wrongfully punished by the law. An example of this latter motive is to be found in "The Adventure of the Blue Carbuncle." Holmes has many extremely wealthy clients too. He becomes so famous that he never has to worry about money. This enables him to pick and choose his cases to suit himself. He is in an enviable position, which is one of the reasons people admire him and continue to read about him after over a hundred years. Sir Arthur Conan Doyle, through Watson, frequently explains why Holmes' eccentric, independent, "bohemian" character can cause him to get involved with people and situations when there is no possibility of his getting any reward for his time and trouble. Doyle wanted as much variety in his stories as he could achieve through characters, settings, and puzzles. His stories take his readers all over England and introduce them to all sorts of people.
Sherlock Holmes contributes markedly to the preservation of law and order in his society because he solves cases that baffle others. His keen sense of logic and his astute skill in forensics uncover clues that often are missed. In addition, he is so clever in his disguises that people do not recognize him and reveal information that they would have concealed from a detective.
In "The Adventure of the Speckled Band," for instance, Holmes is not as easily put off as other people. When he is told by Helen Stone that she fears for her life because her sister has mysteriously died with the last words of "The speckled band!" Holmes agrees to take the case and visits the bizarre home of Sir Grimesby Roylott, who keeps exotic pets such as a cheetah and a baboon. Undeterred by Sir Grimesby's warnings to stay away, Holmes visits first the courthouse where he peruses the mother's will, and then to the countryside where he inspects the room previously occupied by her sister, but now Helen's bedroom. Inside, Holmes detects a bed anchored to the floor, a bell cord that does not work, and a ventilator hole between Helen’s room and that of Roylott.
That night with Watson, Holmes attentively waits in the dark. They detect a slight noise coming from the ventilator and see a dim light. After lighting a candle, Holmes discovers on the bell cord the “speckled band”—a poisonous snake. Hitting it with a cane, the detective drives it back through the ventilator passage; it is an irritated snake that returns to Roylott, who has sent it to kill his daughter, but it strikes him. At this point, Holmes reveals to Watson that after having read Lady Roylott's will, he deduced that Sir Grimesby plotted to kill his daughters and claim the inheritance that his wife has left for the girls.
In another story's exposition, "The Red-Headed League," Sherlock Holmes discusses a case with his friend, Dr. Watson, contending that he will convince Watson of the facts: "...your reason breaks down under them and acknowledges me to be right." Moreover, Holmes guides himself to the solution of his cases because he is "able to guide [him]self by the thousands of other similar cases which occur to [his] memory."
However, in this case, Holmes admits that there are "extraordinary" details. Nevertheless, the ingenious and perspicacious Holmes is able to deduce what will happen in time to avert a major crime. First of all, he invites Watson to a concert, but goes by the assistant to Mr. Wilson and looks at the knees of his trousers. Then, after the concert, Holmes uses his brilliant reasoning power and, having called up Mr. Jones of Scotland and Mr. Merryweather, the bank director, he takes his group to a vault under the bank floor where they wait in the dark. All the suppositions of Holmes are correct: John Clay, the assistant to Mr. Wilson breaks into the area in order to steal French gold that is stored there and is apprehended. Holmes has figured out, also, that Clay would strike on Sunday when the bank is closed.