The Custom House intro, as far as I can tell, was simply a desire of the author to include another story in the same book. I could be wrong-- I haven't read the book yet-- but aside from just providing a frame, which he could've done in a page and a half instead of forty, the most important things in the intro seemed to be sketches-- of him, of his coworkers, of his predecessor, that were maybe not so much relevant as interesting. The book mentions in one of the footnotes that originally there were more sketches included in the book, which he took out, presumably, because they were too irrelevant.
I suppose, though, thinking about it, the sketches, the intro, and when he talks about his connection to Salem, could all just be a way of setting the scene, so to speak.
Like I said, I haven't read the book yet, so I'll reserve any final judgement for next week, or so.