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.