The frame story provides a rationale for the ancient mariner to tell his fantastic tale, while starting from the point of view of the wedding guest aligns us with the normal world of an everyday outsider hearing a bizarre and supernatural tale for the first time. The frame makes it as if we, the readers, are standing beside the wedding guest, listening as spellbound as the guest to the mariner's story. By the end, too, we know from the frame story that the mariner must find people to hear his tale so the he can spread his message:
He prayeth best, who loveth best
All things both great and small;
For the dear God who loveth us,
He made and loveth all.
This helps us makes sense of the adventure: it has a moral and theological heft. Coleridge is suggesting we all should pay attention to caring for the planet and its creatures.
Both the internal rhyme and the alliteration help add a pleasing sense of rhythm to the poem. Some examples of internal rhyme are as follows:
here beat his breast.
Here, the word guest, which rhymes with breast, falls on the fourth beat of the line, halfway through. The same is true below:
The ship drove fast, loud roared the blast.
Alliteration also adds a pleasing sense of rhythm and draws emphasis to important words in a line, as in
The death-fires danced at night
Was a flash of golden fire.
The repeated, alliterative l sounds below reinforce the monotonous, robotic sense of the lifeless limbs:
They raised their limbs like lifeless tools.
All of these literary devices add order and structure to the poem, underscoring the idea that it is not simply a random tale but one with meaning and purpose.