This Frost poem is four stanzas long, and each stanza is four lines long. That's not a very long poem, and despite the title specifically mentioning them, the woods are not talked about very much. We get information in the first stanza that the woods are filling with snow, and then the final stanza tells readers that they are "lovely, dark and deep."
The line containing "lovely, dark and deep" is the line that I think the question is referring to. Structurally, those terms help to keep the rhythm and meter of this poem. The poem is written in iambic tetrameter, and that means the syllables follow an unstressed/stressed pattern. By choosing those three particular words, Frost is able to really drive home the rhythm to readers. A person reading this poem out loud can hardly fail to notice the "da DUM" rhythm of the line.
The woods are lovely, dark and deep . . .
These words also provide readers with the repetition of the "d" sound. We get it in three words in this single line, and it brings a nice sense of euphony to the poem.
Looking beyond the structural implications of those terms, the words create a positive association about the woods. Often, woods and forests are seen as dark and scary. Children's stories are filled with examples of how weird and evil creatures live in forested locations. "Hansel and Gretel" is a good example of this idea. The woods are where the cannibalistic witch lives. That's not the case in this poem. Readers are presented with a forest that is "lovely." It seems to draw the man toward it in a good way. It's a peaceful location, and the woods offer rest and respite. It seems as though the man wants to go that way, instead of toward the town/village in order to keep his promises. It's very reminiscent of the Romantic poets and their love for nature.