Final answer:
A story within a story in 'The Lagoon' deepens themes and characters, creates intrigue, and involves the reader more deeply by layering narratives.
Step-by-step explanation:
The use of a story within a story, also known as a frame narrative, in Joseph Conrad's 'The Lagoon' serves multiple purposes. Firstly, it creates a layering effect that can add depth to the themes and characters by allowing the audience to see how different narrative levels interact. For example, the inner narrative often reflects or contrasts with the outer one, which can heighten readers' understanding of the overall message or moral of the story. Second, it can serve to create a sense of mystery or intrigue, as the inner story is often revealed gradually and may be essential to understanding the outer story. Lastly, it can also help to involve the reader more deeply, as they follow the characters into the story within the story, drawing them in and making them an active participant in the unfolding drama.