Why does Richard Wilbur use envelope rhyme in "Advice to a Prophet?" In other words, what effect does using the "abba" rhyme scheme have on the poem—and how does it support the tone of the poem?
It is impossible to say why any author, especially a poet, writes as he (she) does. He writes about his personal experiences, which the reader most often has no way of knowing about. So, these are my impressions.
In Wilbur's poem, "Advice to a Prophet," what seems apparent to me first is that there is no meter—no discernible, repetitive beat. There is no "sway" in the words; there is no cadence of a march. The meter is all over the place, which might structurally suggest the chaos of war.
In reading the lines and observing the rhyme scheme Wilbur chose, it seems that the lines ending to create the "a" sound (rhyming in lines one and four of the stanza) are straightforward and speak to the heart of the stanza's point. The lines that end with a sound represented by "b" seem to be supportive in nature...not as direct or essential.
The general thrust of the poem is the poet exhorting a prophet not to come and warn everyone about weapons and their massive potential to destroy with overwhelming devastation. The poet notes that mankind cannot conceive these kinds of facts—people have no basis upon which to weigh, compare and/or "appreciate" the prophet's message. The author goes on then, to suggest that the prophet speak about things mankind is more familiar with: nature! And the central ideas he presents in the "a, a" lines seem to go straight to the heart of his overall message.
In other words, the first and last line of the stanzas often seem to fit together, as if a dependent clause had been removed from the middle (lines b, b), leaving the essence of the message behind. Note:
Speak of the world's own change. Though we cannot conceive
How the view alters. We could believe,
These are lines from the same stanza: the first and fourth lines, that have the same sound at the end. These lines seem to fit together so as to almost create a coherent idea without the "b, b" lines. The author might be saying—
...describe how the world will change. While we cannot conceive how the "overall view" of the world might change with weaponry, we could believe (understand) if our world, nature, were to change. We know about nature; we'll understand your words if you connect them with what we already know.
It goes on:
If you told us so, that the white-tailed deer will slip...
The jack-pine lose its knuckled grip
On the cold ledge, and every torrent burn...
The dolphin's arc, the dove's return,
These things in which we have seen ourselves and spoken?...
Dispelled, that glass obscured or broken
In which we have said the rose of our love and the clean...
And all we mean or wish to mean.
Ask us, ask us whether with the worldless rose...
When the bronze annals of the oak-tree close.
The combination of these "a, a" lines seem to create the bare bones of the Wilbur's message. He is suggesting to the prophet that he get to the heart of the matter in a way that people can understand ...otherwise his words will go unheeded.
The tone is how the author feels about his subject—here, very strongly about communicating the effects of weapons. The poet seems to tell the prophet not to waste time describing what cannot be understood. He is telling him to focus on what can be understood. If the "a, a" lines of the rhyme scheme carry the major thrust of the poet's message, then the poem's structure also supports the poem's message, by ignoring the "b, b" lines.
His final point: talk about how the strong, "bronze" oak will change. They will understand that!