This poem is a Petrarchian, or Italian, sonnet. Because it is that kind of sonnet and not a Shakesperian sonnet, it has a rhyme scheme that divides the first eight lines (the octave) from the last six lines (the sestet). By dividing the poem up in this way, the rhyme scheme helps to emphasize the theme.
The theme of the poem is loss -- the loss of things and feelings that one has known in the past. The rhyme scheme separates the poem's discussion of this into two parts. The first part uses the metaphor of lovers while the second part uses the metaphor of a tree and seasons.
By dividing the poem like this, Millay first sets out the theme and then uses a second metaphor to expand on it and emphasize it.