What is the general purpose of a refrain?
Poets use refrains, or repeated lines, most often placed at the end of a stanza, to reinforce the main theme or point of a poem. Though we now most often read poetry from a printed page or a computer screen, the genre arose from an oral tradition. In past times, when most people were illiterate, and before the printing press made widespread distribution of books possible, poems were recited before audiences in public or private spaces. The refrains, because they were repeated over and over, became easier for listeners to remember. This tradition has persisted to the present day. For instance, relatively modern poems use refrains, such as Dylan Thomas's "Do not go gentle into that good night," in which the speaker repeats the refrain "rage, rage against the dying of the light," advising his father and people everywhere to fight hard against dying. Even though we generally read such poems to ourselves, the refrains nevertheless lodge themselves in our memories.
When you add a refrain to a song, or to a poem (particularly ballads), what you are doing is repeating a word, line, verse, stanza, line, or chorus, in order to balance the rhythm of the piece and to even-out the way it sounds. It will sound more rhythmic when you add a refrain.
Check out more information in the link provided on the actual meaning of refrains and some samples of them.
Refrains are usually repetitions of a single word or phrase, even an entire stanza, for the sake of asserting its importance while enhancing the meter or rhythm of the literary work itself.
When an author really wants you to pay attention to a certain point or set of words, he/she will use a refrain to make it obvious. Refrains make reading more choral, or more musical, as the case may be.