From enotes, here's a partial answer to your question concerning Frost's "The Road Not Taken":
Frost composed this poem in four five-line stanzas with only two end rhymes in each stanza (abaab). The flexible iambic meter has four strong beats to the line. Of the technical achievements in “The Road Not Taken,” one in particular shows Frost's skill at enforcing meaning through form. The poem ends:Two roads diverged in a wood, and I— I took the one less traveled by, And that has made all the difference.
The indecision of the speaker—his divided state of mind—is heightened by the repetition of “I,” split by the line division and emphasized by the rhyme and pause. It is an effect possible only in a rhymed and metrical poem—and thus a good argument for the continuing viability of traditional forms.
The repetition of "I", then, separated by the dash and the line break, and emphasized by the rhyme, mimics the indecision of the speaker's mind. The repetition creates the effect of a pause. Notice that "I" is stressed the first time it is used, but unstressed the second. I believe that is what the above speaker suggests when he says that this effect is possible only in a "metrical poem," although he/she doesn't specify.