A scanner records which way up it thinks the page is. When it guesses wrong, every reader shows the document sideways, and rotating in the viewer does not save.
Most PDF viewers let you rotate the view, but that is a temporary setting for your screen. The instruction inside the file is unchanged, so the next person to open it sees it sideways again.
Changing it properly means rewriting that instruction, which is what this does. Nothing is re-photographed and no quality is lost.
Double sided documents scanned in two passes often come out interleaved as well as rotated. The same tool moves pages, so you can fix both in one go.
If every page is on its side, press Turn every page once instead of turning them one at a time. Scans from a feeder often come out with some pages upright and some not, and the thumbnails make those easy to spot. Turn only the ones that are wrong.
Look through the thumbnails before you save. A page that is upside down needs two turns, and a page that is blank can be removed from the same screen. The turns are stored in the pages themselves, so the document will look right in any viewer.
No. Only the order and the which way up instruction change.
Yes, remove it and it simply will not be in the file you save.
Yes, the thumbnails and controls work on a touch screen.
Use the Turn every page button. Each press adds a quarter turn to all of the pages, so press it again for a half turn.