Each character in a story is included to serve a purpose, whether it is to move the plot forward or to serve as an antagonist to the main character. Clarisse serves several purposes in this novel. She is a teenager, representing innocence and imagination, and is the antithesis of what Montag's society is all about. She provides Montag with the motivation to question his society and his own role in furthering the goals of that society. Clarisse and her family represent what Montag's society could and should be. They talk to one another and care about each other. They aren't lonely or alienated from one another.
Why does Clarisse have to die? Her death emphasizes all that is wrong with Montag's conformist society. Bradbury uses her to show the inhumanity of the society in his novel. If a society would kill a sweet, innocent girl, what does this say about it? Her death intensifies Bradbury's message to the reader.
The death of Clarisse not only shows everything that is wrong with the Fahrenheit 451 society, but it also gives Montag an eye opening experience through which he begins to make his own choices. Her death brings questions that Montag had toyed with in the past to an almost self-inflicted obsession that causes him to act. It is only after her death that he begins to read openly and to confront Mildred, the fire chief, and the rest of the society.