A booklet is a staple of pages that is folded in the middle to be used like a book. This means the pages have to be resorted before printing and two pages have to be shrunk onto one. Create Booklet does that for you and as it is a PDF Service you can do it directly from the print panel. Additionally you get the Automator Action that is used in that PDF Service.
What is new in this release:
Snow Leopard Update
Right to Left support in Automator Action