"SOMETHING there is that doesn't love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun;
And makes gaps even two can pass abreast."
(Ln 1-4)

The absence of rhyme in this poem is definitely necessary. The poem is meant to feel lonely and quiet, if there was rhyme in the poem, it may feel like a peaceful quiet or at least less lonely.

Absence of Rhyme