A wedding guest who does not know the mariner is forced to listen to his tale. Why does Coleridge employ this device? Is the guest meant to guide the readers response to the mariners tale?

Coleridge employs the device of a story within a story. First we have the Mariner and the Wedding Guest, then we have the tale of the voyage. The Wedding Guest's place as the listener is explained in Part Seven when the Mariner tells the guest the story of the Hermit.

I think it is becaue of knowledge.