The two-pass scan, and why it is such a nuisance
Scanning a double-sided document on a single-sided feeder means running the stack through once for the fronts, flipping it, and running it again for the backs. What comes out is pages 1, 3, 5, 7 followed by the even pages — and because flipping the stack reverses it, usually in descending order: 8, 6, 4, 2.
Fixing that by hand on a twenty-page document is forty drag operations, and one slip puts you back at the start. It is the single most common reason anyone opens a page-reordering tool, and no popular tool has a button for it.
“Fix duplex scan” is that button. It interleaves the first half with the reversed second half in one press. If your scanner produced the backs in forward order instead, “Interleave halves” handles that variant.