Arthur Conan Doyle used Dr. Watson as the narrator in almost all of the Sherlock Holmes stories he wrote. He must have given a lot of thought to this technical matter. He was following the example set by the great American literary genius Edgar Allan Poe in his "tales of ratiocination," notably "The Purloined Letter," "The Murders in the Rue Morgue," and "The Gold Bug." F. Scott Fitzgerald later did the same thing in his novel The Great Gatsby, which is narrated by Nick Carraway although the hero is Gatsby. By using what is commonly a "minor character as narrator," Doyle had the freedom to insert extraneous description of such things as the landscape, the weather, and the overall mood. This would have been difficult if Doyle was inside the detective's mind and trying to focus on his thought processes. Typically, Holmes will explain his deductions at the end of the story, and usually he will explain them to his friend Watson after the action is all over. This is the case, for example, in "The Red-Headed League." Poe does the same thing in the three stories mentioned above. The more dramatic elements in the stories come first, while the explanations come last. It would seem to be nearly impossible to have both the dramatic and the ratiocination come at the same time. Watson is also free to describe Holmes in action, and inaction, and to express his great admiration for his friend. Holmes could hardly describe himself objectively or express his admiration for his own brilliance.
Raymond Chandler, one of America's best mystery writers, narrated his novels in the first-person from the point of view of the detective Philip Marlowe. Marlowe tells the reader some of the things he observed--but not everything. In The Big Sleep, for example, he does not tell the reader how he guessed that Carmen had murdered Rusty Regan until he springs it on Vivian Regan at the end. And in Farewell, My Lovely, he does not tell the reader that he guesses that Helen Grayle is the Velma that Moose Malloy is seeking until Moose actually identifies her towards the end. In the aftermath Marlowe explains all his ratiocination to Anne Riordan at her home in Bay City over a few glasses of whiskey.