I use Preview’s ability to manage pages in PDF files after scanning pages into the computer (points number two and three). Ofttimes the back side of the page gets scanned in (I frequently use green engineering paper twenty years after college) and I want to remove those from the PDF. I also will merge pages from PDF. Drag. Drop. Save. Done.
I ran across a Macworld article called Preview’s Hidden Powers that lists of the capabilities of the built-in Preview tool in OS X. It lists a couple of other uses that you might find useful.