In the poem "The Road Not Taken" by Robert Frost, why does he use literary techniques to reveal journeys?

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.