First, a word about the nature of poetry. This question—"How do poetic devices reinforce the speaker’s intention in the poem?"—demonstrates a major error in thinking about poetry, or any work of art. The speaker/poet’s “intention” can not be known; even if the poet makes a statement claiming an “intention,” readers/listeners cannot take such a confession as gospel: the only testimony is the poem itself. Readers/listeners of a poem can experience only what the poem is doing, not what the poet/speaker of the poem intended, or may claim to intend, it to do.
Second, poetic devices are often used in poems to communicate on, a figurative level, thoughts, emotions, events, and ideas that are ineffable and thus cannot be communicated on a literal level. Thus, poetic devices could never perform the function of “reinforc[ing] the speaker’s intention.”
Third, regarding Frost’s “Stopping by Woods on a Snowy Evening”: This poem remains quite literal. Its use of poetic devices is limited to rime and meter. The rime scheme is AABA CCDC DDED EEEE; the meter is iambic tetrameter.
(Please note: The spelling, "rhyme," was introduced into English by Dr. Samuel Johnson through an etymological error. For my explanation for using only the original form, please see "Rime vs Rhyme: An Unfortunate Error at https://owlcation.com/humanities/Rhyme-vs-Rime-An-... .”)