The most likely reason Frost includes Assonance in the excerpt lines is; To create pleasing sounds in the second line.
What is Assonance?
Assonance is a literary device characterized by the resemblance of sound between syllables of nearby words, arising particularly from the rhyming of two or more stressed vowels, but not consonants, but also from the use of identical consonants with different vowels.
On this note, the use of Assonance in the excerpt lines is in a create pleasing sounds in the second line by matching shout to about.
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