Bradbury uses different characters in the novel to make readers aware of the need for a purpose in life. Mildred is the most stark example of a person who suffers from not having any sense of purpose. Early in the novel, her sense of meaninglessness becomes so profound that she tries to kill herself by overdosing on pills to escape a life of mindless, empty television watching. However, when she returns to life, she is afraid to leave the security and conformity of her telescreens and lapses back into a purposeless life. When she dies, Montag can hardly grieve her, because it feels as if she hadn't been really alive.
She is contrasted at the beginning of the book to the vibrant, acutely alive Clarisse, who walks in nature, notices what is around her, and actually engages people in conversation. She listens to and is interested in what Montag has to say. Her family talks and takes walks rather than watching television.
Likewise, at the end of the novel, Granger remembers his grandfather, who, unlike Mildred, had a strong sense of purpose and engagement with life. He was sculptor who played the violin, told jokes, helped clean up a slum area, and raised pigeons and doves. He made an impact on the world around him. When he died, people missed him because he had been so fully alive and motivated to be an active part of his society.