A booklet requires imposition. Pages are reordered in a non-obvious way: one sheet carries, say, pages 8 and 1, with 2 and 7 on the reverse. After duplex printing, folding in half, and nesting the sheets, you get a booklet that reads in the correct order. Working that sequence out by hand is painful and error-prone – which is precisely why the tool does the layout.
Multiple pages per sheet (2, 4, 8, 16) is far simpler: pages are scaled down and tiled in normal reading order with no reordering at all. This is the mode for drafts, handouts, and saving paper on long documents. No folding is implied.
Things to watch when printing a booklet. You need duplex printing that flips on the short edge – flipping on the long edge puts the reverse side upside down. The total page count must be a multiple of four; missing pages are padded with blanks automatically. And there is an opposite case – spreading a single page across several sheets – which is poster printing, not a booklet.