I might use this for better margins when printing PDFs I didn't generate for bookbinding; it's not rare PDFs (research papers, etc) are rendered with shallow margins for full-page print, maybe with a deeper gutter to allow for three-hole punching, but a perfect-bound book requires deeper margins all around to account for trimming and since PDF doesn't reflow, downscaling is the only feasible option.
Granted, this is something I already have a workflow for, so I don't actually need this particular tool. But if I were assembling such a workflow, I might well consider this as part of it.
I have not used this specific tool, but have used https://community.coherentpdf.com/ for the cropping/rescaling. And in general "printing" (posters) is what I have used it for (books make sense though as well).
Depending on the site and what you are doing it may auto-rescale, I try to make it exact though just to prevent any nonsense on their end (and in Adobe PDF you can print tile to check it out as well in actual size).
You might want to make the content of a pdf as big as possible (e.g. to zoom/scroll less on an eink device). This tool could help, you could use it to remove margins or otherwise crop.
