You ask why Frost uses "literary techniques to reveal journeys." You don't ask how he does it or what the results of what he does are, so I will specifically answer the why.
First, why does he use literary techniques to reveal journeys? Because that's what poets do. Metaphor, in a broad sense, is at the heart of most writing, but especially poetry. Human beings tend to think in metaphorical terms. Comparison is one basis of our knowledge and how we learn. Metaphor in poetry allows readers to feel a sense of discovery as we "decode" the metaphor, and readers love to think we are making discoveries.
Using metaphors gives layers to the ideas poets express. Metaphors have two parts: tenor and vehicle. The tenor is the idea the poet wants to communicate (decisions made in life, for Frost, here), and the vehicle is whatever is used to help explain the tenor (the fork in the road, for Frost). Frost uses metaphor because it works. It's a better way to explain.
There are other literary techniques used here, of course, but metaphor dominates the poem.
I think that you want to analyze the language Frost uses in the poem that helps to convey that freedom and the act of choice can represent its own journey or voyage. Note in the first stanza, the use of terms such as "traveller" and "travel." The roads themselves might not represent this journey, as much as the analysis of which path to take. In this instance, the voyage is the process and not the product. The next two stanzas go through the criteria that helped the speaker make the choice of which one of the two paths to take. Again, this might reflect how the real journey is the choices made upon them and not anything else. Freedom and choice are journeys, voyages into the hearts and minds of those who have to make such decisions, and Frost might be trying to explain this through the language selected.
This poem explores the idea that every choice we make determines what we are and what we will be. The two roads symbolize choices of direction—career, personal involvements, activities, development of character. Stanza 1 sets up the circumstances of the choice; the roads diverged and the speaker was sorry he could not choose both.
Stanza 2 suggests that the two roads (choices) were almost equal, but there is a difference. The speaker picked the one that wanted wear and was less traveled (less conformist, less traditional, less popular).
Stanza 3 takes up the inevitable closure produced by choice; the speaker saved the unchosen road “for another day,” but he knew that he would never “come back” (face the same choice again) because “way leads on to way” (each choice leads on to subsequent choices). All this is in the past. The choices have been made and the speaker’s life has been set. In the last stanza, the speaker considers the impact of such choices on the future; although one of his choices of direction may have seemed insignificant at the time he made it, this choice will have made “all the difference” in life.