Why does the poet mention where woods have burnt?

Robert Frost's "A Passing Glimpse" is a poem about perspective, and the example of the burnt woods is one of several that he includes to support his central idea.
The poem begins with the speaker commenting that "I often see flowers from a passing car / That are gone before I can tell what they are" (1-2). These lines introduce the idea of perspective. He sees the flowers as a blur as his train speeds by. He cannot see them in enough detail to identify them, but for most of the remainder of the poem, the speaker describes what he might see if he were to disembark from the train and explore the flowers up close. In his imagination, he says that he "name[s] all the flowers I am sure they weren't" (5).
The reference in your question is the first example he states: "Not fireweed loving where woods have burnt" (6). Fireweed grows in the space where trees have previously burned, thriving on that damaged ground. Frost's speaker qualifies this by saying it's actually not what he would've seen. He also would not have seen bluebells nor lupine. He cannot match up what he thinks he might have seen with what is actually there. He thinks he may have stumbled upon something no one else has ever seen (9-10). The poem's final two lines are the speaker's summation about this theme of perspective: "Heaven gives its glimpses only to those / Not in position to look too close." This seems to indicate that it's not meant to be that we see the same things from a distance and up close. There seems to be some inherent injustice to the sentiment in the final lines, though: only those who cannot appreciate the view are given the perspective.