Why does the poet use the expressions "dark," "deep," and "darkest"?

The words “darkest”, “dark”, and “deep” are each used once in Frost’s poem to create a complex atmosphere of the narrator’s experience. The stop in the woods occurs on the darkest night of the year, invoking what might be a frightening experience. However, the narrator mentions that he might know the proprietor of the woods, which softens the danger of the lack of light.
Later in the poem, the woods are referred to as both "dark" and "deep", again referencing what could be a solitary and fearful visit. Who knows what lies beyond the immediate? Dark and deep could also refer to the sleep that is referenced a few lines later, possibly reinforced by the “easy wind and downy flake” of the blanketing snow. The night might be extreme (darkest), but it is not alarming, and while the narrator seems to be quite tired, he is looking forward to a rest as equally "lovely, dark and deep" as the forest upon his homecoming.

Frost uses these descriptive terms to create a sense of atmosphere. The word "darkest" is applied to the evening: this term is a superlative, meaning that something that is "darkest" is darker than anything else. Because this is not only a dark evening, but the darkest of the year, then, Frost is emphasizing the strangeness of being out on such an evening, stopping in the woods.
In the final stanza of the poem, Frost reiterates the fact that the woods are "dark," but on this occasion it is clear that the darkness is not something to be feared—on the contrary, the fact that the woods are both dark and deep seems "lovely" to him. This creates a sense that the dark depths of the woods are something comforting, enfolding, like the blanket of snow which covers them. The speaker creates a haunting sense of place with these terms: we picture him and his horse enjoying the dark quietude of the woods, and not wanting to leave, and yet being drawn ever onwards by the "promises" they have to keep.