Chapter Fifteen is basically an overview of the case, tying up any loose ends or questions that might still be lingering in the reader’s mind. This method of wrapping up is used frequently in detective stories, up to the modern day television show Monk. Clues are dropped at various points throughout the story but the reader never has enough knowledge to completely solve the case until this point, when the detective sums up the case.
This is unlike most other genres, where readers are used to finding things out as the characters do. But as Watson noted in Chapter Fourteen, detectives, unlike other people, do not like to disclose information beforehand; and Holmes would be the last to give up his talent to “dominate and surprise those who were around him.”