Let's take two random half lines, and just stick them together, like this:
an army was fleeing
Two such half-lines, as they stand, are almost indistinguishable from prose.
If we were writing free verse, that would not, perhaps, matter too much.
But what we are discussing
is structured verse, not free verse, and it needs to have a clearly measured rhythm, and a pattern
which creates a kind of momentum as the poem is read aloud. And alliteration is the
device used to measure out this rhythm.
What is alliteration?
Essentially, alliteration is repetition of beginning consonant sounds.
For example, we could change the first half line above to make it alliterate,
an army was driven
The alliterating consonant is D. The repetition of this consonant at the
start of driven and deep ties the two half lines together,
increases emphasis on these two key words, and builds a stronger rhythm.