Repetition is the basis for many poetic forms. The use of repetition can heighten the emotional impact of a piece. Repetition of a sound, syllable, word, phrase, line, stanza, or metrical pattern is a basic unifying device in all poetry. It may reinforce, supplement, or even substitute for meter. Repetition emphasizes whatever it is that is repeated, making it stand out so the reader knows it is important. If you repeat a word or a line in poetry, then that word or line (or those words or lines) appears to be more important than other parts of the poem. In fact, in a poem with repeating lines, all of the other lines are often comments on or elaborations of the repeated line. Repetition can also affect the rhythm of a poem and the way it sounds. In particular, repetition of individual sounds or groups of sounds can strengthen the rhythmic structure.
Repetition can be a great tool in poetry and a very effective one, when done well (right), but it is not easy to do. Anyone whose ever partook of poetry forms that use repetition know that it can be a difficult task, that repeating lines and phrases dispersed throughout your poem can sometimes adversely affect your rhythm and the overall flow of the poem. Repetition is not the hardest thing in the world to do, but is very easy to do badly.
Repetition emphasizes whatever it is that's repeated, but too much repetition can make a great word or phrase seem commonplace. It's a matter of balance or moderation. Repetition is another one of those elements that we usually think of in connection with strict forms of poetry, but which is also of great use in less structured poems, including free verse. There are many possibilities--one can repeat words, phrases or whole stanzas, and one can play with the location of repeated parts. One of the keys to repetition is what you choose to repeat and where you choose to repeat it. If it's done in a poem that requires a specifically structured repetition, as many of the repetitive poetry forms/formats do/require, then it's a matter of effectively choosing what will be repeated in your poem, choosing what to use as your refrain or repeated sound, syllable, word, phrase, line, or stanza one that will still allow it to be rhythmic and fluent (to flow).
If repetition, repeating yourself, in a poem, doesn't lend itself well to the rhythm and flow of the poem, it shouldn't be done, used, at all. I personally have tried using repetition effectively in poetry and can safely say doing it "effectively" is the most difficult part of the task, although I have managed to do so on occasion.
Repetition is a skill and like all skills not everybody is graced, blessed, with the ability naturally. The use of repetition is an investment of time and effort. It takes a certain level of skill and craftsmanship to do it well, so for those who aren't graced with the natural ability it takes work to be able to do it well, right, if you will.
Some of the information from an internet article on the Elements of poetry that is no longer up.