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.