In the poem "Writing in the Afterlife" by Billy Collins, what insights does he offer about writing and revision?

In “Writing in the Afterlife,” Billy Collins compares eternal suffering in the afterlife to the process of writing. The afterlife in Collins's poem is the afterlife of Greek mythology—complete with river and ferryman. The poem sets the idea of the afterlife as being an unpleasant place that is filled with people on boats, never reaching the other side, furiously writing under the guidance of a Chiron that is always chiding them to do more.
Collins makes writing and revision seem unpleasant. He adds in the pedantic voice of the writing teacher to chide the tormented souls on the boats,

Our next assignment would be
to jot down, off the tops of our heads,
our thoughts and feelings about being dead,
not really an assignment,
the man rotating the oar keeps telling us—
think of it more as an exercise . . .

The teacher is continuously scolding them to do more things. The tone of the piece is depressing, brought on by the almost sarcastic comments of the boatman. The idea that it's not an “assignment” but an “exercise” implies that it's for the good of those trapped souls, but it is clear from the language that it isn’t done by choice. It's a chore and a struggle rather than something people would prefer to do.
The idea is that the writing process is like a prison—something people must continually do against their wills. The boatman has a candid moment in the poem where he says,

Think of writing as a process,
a never-ending, infernal process . . .

Writing is like eternal, infernal torture.