A manuscript exported straight from a word processor is usually the wrong shape for a printed book. It is letter or A4 sized, its margins are the same on both sides, and it has stray blank pages where sections ended. This tool reshapes that file into a print interior.
It sets every page to one trim size, applies margins appropriate to a bound book, removes blank pages you do not want and resamples oversized images so the file is not larger than it needs to be.
Automated formatting suits text-led books: novels, non-fiction, workbooks with a regular structure. It is the wrong tool for a heavily designed interior, where a page is a composition rather than a container for flowing text.
It standardises geometry: page size, margins, blank pages and image weight. It does not make typographic decisions about spacing, hyphenation or chapter openings, which is what typesetting means.
The one your book will be published at, decided before formatting rather than after. Reformatting to a different trim later reflows the interior and changes the page count, which changes your spine and print cost.
It suits text-led interiors with a repeating structure. Books where each page is individually designed, such as illustrated children's books or complex workbooks, are better laid out by hand.