Alliteration enhances the poem's rhythm and accentuates or intensifies several key passages.
In lines 1-3, for instance, /dr/ and /b/ alliteration creates cacophony, underscoring the discomfort of the speaker's "drive":
As I drive to the junction of lane and highway,
And the drizzle bedrenches the waggonette,
I look behind at the fading byway,
These sounds also appear as consonance: the /dr/ sound in "bedrenches" and the /d/ sounds in "behind" and "fading" add to the harshness of the passage. That harshness, in turn, helps evoke the speaker's grief upon revisiting this place.
By contrast, the gentle /s/ alliteration of lines 9-10 seems to mirror the relief the lines describe:
To ease the sturdy pony's load
When he sighed and slowed.
Later, in lines 14-15, Hardy uses /r/ and /f/ alliteration back to back:
Without rude reason till hope is dead,
And feeling fled.
Here, the harsh /r/ and fricative /f/ sounds seem to fit the lines' content, which involves the "dea[th]" of "hope" and the loss of all "feeling." These emphatic sounds add a little extra force to Hardy's passionate pronouncements. Similarly, in line 24, /c/ alliteration ("colour and cast") intensifies his claim that his passion for Emma Gifford somehow imprinted the landscape itself.
The sixth stanza contains various alliterative sounds, which together highlight Hardy's supernatural vision of his late wife. Harsh /r/ sounds ("rigour," "rote," and "ruled") help convey how cruel and relentless time is. Fricative /f/ sounds ("phantom figure") help impress the haunting memory of Gifford on readers' imaginations. Smooth /s/ alliteration ("sight," "substance," "slope," "Saw") adds to the hushed intensity of the scene.
Finally, the /s/ alliteration in line 33 ("sand is sinking") underlines Hardy's metaphor about his waning time on earth. In a subtle, muted way, it evokes the slippery swiftness of time's passage.