Theory:

What is Repetition?
Repetition is an act of repeating a word or phrase. It is commonly used to stress a point or to make a statement. It also helps in enhancing the general tone and style of the literary text. Hence, when used, repetition modifies both the form and the meaning of the text.
Definition:
According to J. A. Cuddon, repetition is “an essential unifying element in nearly all poetry and much prose. It may consist of sounds, particular syllables and words, phrases, stanzas, metrical patterns, ideas, allusions and shapes. Thus refrain, assonance, rhyme, internal rhyme, alliteration and onomatopoeia are frequent in repetition.”
The following is a famous speech from Shakespeare's Julius Caesar, spoken by Mark Antony. Shakespeare has used the rhetorical device known as repetition to deliver the message. The lines "Brutus says he was ambitious" and "Brutus is an honourable man" are examples of repetition, and one can easily see how the repeated lines had a positive impact and effect on the outcome of the speech.
Example:
Friends, Romans, countrymen, lend me your ears;
I come to bury Caesar, not to praise him.
The evil that men do lives after them;
The good is oft interred with their bones;
So let it be with Caesar. The noble Brutus
Hath told you Caesar was ambitious:

If it were so, it was a grievous fault,
And grievously hath Caesar answer’d it.
Here, under leave of Brutus and the rest–
For Brutus is an honourable man;
So are they all, all honourable men–
Come I to speak in Caesar’s funeral.
He was my friend, faithful and just to me:
But Brutus says he was ambitious;
And Brutus is an honourable man.

He hath brought many captives home to Rome
Whose ransoms did the general coffers fill:
Did this in Caesar seem ambitious?
When that the poor have cried, Caesar hath wept:
Ambition should be made of sterner stuff:
Yet Brutus says he was ambitious;
And Brutus is an honourable man.

You all did see that on the Lupercal
I thrice presented him a kingly crown,
Which he did thrice refuse: was this ambition?
Yet Brutus says he was ambitious;
And, sure, he is an honourable man.

I speak not to disprove what Brutus spoke,
But here I am to speak what I do know.
You all did love him once, not without cause:
What cause withholds you then, to mourn for him?

O judgment! thou art fled to brutish beasts,
And men have lost their reason. Bear with me;
My heart is in the coffin there with Caesar,
And I must pause till it come back to me.
The above speech has got various kinds of repetition in it, such as refrain, polyptoton, and anadiplosis. Apart from emphasising the meaning, the technique of repetition had also created an impact in the style by giving it rhythm.
  
Hence, repetition can affect both the form of the text as well as its meaning.
Why is the repetition important?
Repetition is important because it helps the speaker or author emphasise the important points. It also helps the audience or readers to understand the meaning of the poem or speech very easily.
Repetition used in the poem "A Legend of the Northland":
Away, away in the Northland,
Where the hours of the day are few,
And the nights are so long in winter
That they cannot sleep them through;
 
Where they harness the swift reindeer
To the sledges, when it snows;
And the children look like bear’s cubs
In their funny, furry clothes:
 
They tell them a curious story —
I don’t believe ’tis true;
And yet you may learn a lesson
If I tell the tale to you.
 
Once, when the good Saint Peter
Lived in the world below,
And walked about it, preaching,
Just as he did, you know,
 
He came to the door of a cottage,
In travelling round the earth,
Where a little woman was making cakes,
And baking them on the hearth;
 
And being faint with fasting,
For the day was almost done,
He asked her, from her store of cakes,
To give him a single one.
 
So she made a very little cake,
But as it baking lay,
She looked at it, and thought it seemed
Too large to give away.
 
Therefore she kneaded another,
And still a smaller one;
But it looked, when she turned it over, 
As large as the first had done.
 
Then she took a tiny scrap of dough,
And rolled and rolled it flat;
And baked it thin as a wafer —
But she couldn’t part with that.
 
For she said, “My cakes that seem too small
When I eat of them myself
Are yet too large to give away.”
So she put them on the shelf.
 
Then good Saint Peter grew angry,
For he was hungry and faint;
And surely such a woman
Was enough to provoke a saint.
 
And he said, “You are far too selfish
To dwell in a human form,
To have both food and shelter,
And fire to keep you warm.
 
Now, you shall build as the birds do,
And shall get your scanty food
By boring, and boring, and boring,
All day in the hard, dry wood.”
 
Then up she went through the chimney,
Never speaking a word,
And out of the top flew a woodpecker,
For she was changed to a bird.
 
She had a scarlet cap on her head,
And that was left the same;
But all the rest of her clothes were burned
Black as a coal in the flame.
 
And every country schoolboy
Has seen her in the wood,
Where she lives in the trees till this very day,
Boring and boring for food.
 
There are two different kinds of repetition used in the poem: epizeuxis and diacope.
 
The repetition of the word "away" in the line "Away, away in the Northland" is an example of epizeuxis as the word is repeated immediately without any word in between.
 
The repetition used in the lines "And rolled and rolled", "By boring, and boring, and boring", and "Boring and boring for food" are examples of diacope as there is a word in between the repeated word.
Important!
Learn more about epizeuxis and diacope with sufficient examples in the following chapters.
Reference:
  • https://www.poetryfoundation.org/poems/56968/speech-friends-romans-countrymen-lend-me-your-ears
  • Cuddon, J.A. A Dictionary of Literary Terms and Literary Theory. West Sussex, Wiley-Blackwell Publication, 2013.
  • National Council of Educational Research and Training (2006). Beehive. A Legend of the Northland - Phoebe Cary (pp. 65 - 67). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.