A plot is necessary to keep a reader interested in a story. Enotes defines a plot as "the order of events in a narrative or any other type of story." The importance of the plot lies in how a story follows chronology. For example, a story may be chronological or have flashbacks or be framed. If an author fails to follow the typical plot line (exposition, rising action, climax, falling action, and denouement, or resolution), a reader may become confused regarding the order of events. Once a reader is confused, the chances of their interest being lost is high.
That said, a plot which builds in intensity and conflict tends to keep a reader engaged and wanting to read more. That said, a story which lacks conflict or rising action fails to climax and the reader may see no reason to continue reading.
Therefore, in order to keep a reader interested, a writer must include interesting characters, developing and deepening conflict, and a resolution which leaves the reader feeling a sense of closure.