Repeated lines often refer to a specific theme or concept that the author wants to be clear. "I fear thee ancient mariner" refers to the Wedding-guest's fear that the Mariner is supernatural. Before the mariner finishes his story, the guest believes that he is a ghost. The mariner must calm these fears. The theme to which this points is the supernatural nature of the story.
I believe the other line that you are asking about is "he cannot choose, but hear." This shows that even though the wedding guest is frightened and slightly disturbed, he cannot leave because he is enraptured and captivated by the story. This line also points to the supernatural effect that the mariner has over the Wedding-Guest.
The repetition of the lines show the importance of the supernatural demeanor that the mariner has.