If we read Heart of Darkness as being a book about a man's attempt to escape human limitations, the frame story can be said to be set reify or define the notion of limits. No matter what Marlowe says, no matter what story he tells the men on the boat, the reality for those men on the boat will not change.
Marlowe's speech is, essentially, just a movement of air. It is just words. Anything he depicts in his story will remain just that, only a story. There are limits on what can be achieved through imagination and fantasy. We see this in Kurtz as well. He becomes obsessed, apparently, with becoming more than human. He seeks to escape the bounds of morality and mortality, and fails. Even a demented imagination has limits as to what it can do to change reality.
If theme here is breeching the unknown, then the frame story again has resonance. The story of Kurtz is one that must be imagined by the people sitting on the boat, listening to Marlowe. In order for them to really "see" the story, they must leave reality behind and engage with fantasy. This parallels both Marlowe's journey into an unreal/unknown territory and Kurtz' attempt at escaping the confines of "known" humanity.
If the themes of this book deal with the unreal and the attempt to breech the unknown, the frame story serves to quickly delineate the real from the unreal and the known from the unknown.