Tennyson uses the last line of "The Lady of Shalott" as a refrain throughout all of the stanzas in the poem.  The significance of his usage of the refrain is two-fold: it heightens the cadence and rhythmic quality of the poem as well as reinforcing the focus of the poem on the central character, The Lady of Shalott. 

Refrain is a commonly used device in poetry, and in "The Lady of Shalott," Tennyson repeats the phrase "The Lady of Shalott," at the end of each stanza, which increases the lyric quality of the verses.  Ultimately, Tennyson's use of the refrain repeatedly draws the reader's attention back to the character of the lady to emphasize that this is her story among the legends of Camelot--not Arthur, nor Lancelot, but rather "The Lady of Shalott."