When I read this poem, I interpret the caesurae (plural for caesura) as slowing down its pace, which makes sense considering the speaker's claim that, when Death came to collect her, "He knew no haste." First, they drive near the school to watch the children play, then to the fields to watch the sun set over the grain. She describes the way night seems to fall after the sun has passed them, and how she grows chilly. Finally, Death's carriage arrives at the speaker's grave "in the Ground," and she describes how mortal time is moving so quickly while time for her moves so slowly because she is measuring it against "Eternity." Therefore, Dickinson's use of caesurae really slows the pace because the reader must pause so often, at least every time she employs an end dash, and this is appropriate to the content of the text.
In addition to slowing the pace, although related, the mood of the poem is made calmer by all of the pauses. Just as Death "knew no haste," neither does the speaker. She, quite literally, has all the time in the world, and she seems perfectly resigned, even somewhat flattered, by Death's calling upon her as if they were lovers and he is simply arriving to pick her up. Therefore, both the word choices as well as the caesurae produce the poem's calm and tranquil mood.