When people ask a rhetorical question, it is often because the question has an implied answer. When the speaker asks, "What passing-bells for these who die as cattle?" the implied answer is none. The question itself is jarring, comparing the soldiers who die to cattle, perhaps because of the sheer numbers of those who die, or perhaps because they die without fanfare—without even being noticed. When the speaker asks, "What candles may be held to speed them all?" the implied answer, again, is none. There are no funeral masses or proper good-byes for these soldiers who "die as cattle" because their bodies may not even be recovered or sent home. They will not get real send-offs because of how and where they perished.

By asking questions with implied answers, Owen draws attention to the all too commonplace nature of death in war—how it happens in such numbers and with such frequency that we cannot even properly honor all those who pass. What kinds of bells or candles should these soldiers get? Really, the best we have to offer, right? And yet they will get nothing; the contrast is so stark as to strike us more deeply.

In "Anthem for Doomed Youth," Wilfred Owen is attempting to convey his opposition to World War I.  Owen wants to show the sorrow that war brings and its utter wastefulness (as it destroys the lives of so many young men).

The use of a rhetorical question to open each stanza is meant to help him accomplish this goal.  By using a rhetorical question to begin the stanza, he invites the reader to think about that question.  This allows the reader to be more of a participant in the poem.

When one wants a reader or listener to truly understand and buy into ones point of view, the use of rhetorical questions is effective because it gets them to think about things for themselves. As an example, teachers know this and often try to ask questions whose point is to get students thinking rather than having them just listening to the teacher's talk.