Symbol Analysis
Okay, so this one's probably a no-brainer. Blake was both a writer and a publisher, so it probably stands to reason that he would talk about this in his work. Usually, when a writer does something like this—when a poet writes about… writing poetry, for example—it's a sign that the idea well has run totally dry. But that's not the case with Blake.
For one thing, Blake was an innovator in pretty much everything he did. He learned engraving as a trade, but then he totally changed the game with his new process. He applied this process when he printed his own work, which means that he knows what he's talking about when he writes: "I saw a mighty Devil […] with corroding fires he wrote" (3.2). Those corroding fires were in fact part of Blake's copper engraving method, and so we can see that this is a sly nod to himself as this Devil. (Check out "Speaker" for more.)
Later, Blake visits "a printing-house in Hell" (6.1), which seems like a strange place on your list of destinations if you're touring the underworld. But the fact that he does include this stop shows us the importance that Blake put on his own daily work. The dragons, vipers, and eagles that Blake describes in this "Memorable Fancy" section are all hard at work, doing jobs with which he would have been very familiar. More importantly, they're stocking Hell's libraries with books, books that contain exactly the kind of Energy and creative force that Blake is arguing for throughout The Marriage of Heaven and Hell.