A plot in a story refers to the events which are connected by cause and effort.
Whereas, subplots exist within the main plot as a side story. It never overpowers the main plot but is connected with the main plot. Events that take place outside the main story are described in subplots. Therefore, the purpose of the subplot is to strengthen the main story line.
Therefore, one reason why subplots are used in novels is to develop the novel's theme in more depth.
