Using a frame story has a number of benefits.
Often, as in the case of Conrad's novel, the frame story situates the central narrative as a "told story". This metaphorically and practically links the novel's narrative to traditions of story-telling and folklore.
The connection with folklore is especially significant as folklore is and was laden with symbolism and charged with the power of myth.
Not only does folklore entertain, but it passes on the culture and behavior models of a people, which psychologist Carl Jung called “the collective unconscious.”
As a literary type, folklore uses representative figures to tell stories that work like allegory - expressing ideas that are meant to communicate social truths, human truths, and ideas that extend beyond the actual narrative.
Conrad's novel can be understood in these terms.
In this somewhat ironic way, using a frame story to present a narrative offers a way to signify artistic intentions that go beyond both the central narrative and the frame story, approaching the depth and breadth of myth.
This rationale certainly seems to apply to Conrad's work as the novel presents an image of the products of European culture run rampant in Africa on ivory expeditions. Kurtz is aligned with Europe and can be seen as a symbol of the continent's ambitions, hubris, and genius.
"All Europe contributed to the making of Kurtz..."
There is no need to labor over the proofs of symbolism in the novel. These proofs abound and exist on several levels, creating a work rich in meaning. The important point here is to acknowledge that the book attempts to communicate something universal; ideas that go beyond the narrative and which thereby use the narrative as a vehicle of folklore or myth, laden with cultural, social and/or human truths.
What these truths are, exactly, is open to interpretation. Critics have pointed to this idea as another way the frame story is useful. With one narrator opening the story and describing Marlow telling a story, we have a series of voices in play, each functioning as interpreters of facts and events. Each voice filters the "truth" in its own way, choosing the details to be included and excluded from the narrative.
These perspectives are often conflicting and are always open to a variety of interpretations. Whose point of view is to be trusted? Which narrator and which character is reliable?
A frame story of the sort used here naturally implies a personality behind the act of storytelling, fallible and potentially biased. When this element of personality is multiplied, the effect of doubt is also multiplied. Yet, there is certainly truth in the text of some kind. The frame story serves to convey at one time each of these conflicting ideas - a universal truth exists in the narrative, but deciphering that truth requires us to get past the personalities involved in the story, which shape that truth.
The enigma we are left with is, arguably, more potent for its mystery than it would be if simplified or exposed.