To transclude a page into another page just add the page reference as a markdown block, that will insert the page into your page. E.g. In Nitoku we use the following markdown block definition to insert the license of all the blocks that we create.
The page transclusion is performed when the referencing document is displayed, and is automatic and transparent to the end user. The result is a single integrated document made of pages assembled dynamically from separate sources, this functionality facilitates modular design: a page is stored in once place and distributed for reuse in multiple documents. Updates or corrections to a resource are then reflected in any referencing documents.
Reusing pages is a basic way of avoiding repeating yourself. E.g, there are more than 40 Nitoku blocks developed by nitoku.com, every block have their license written on the block page, if we didn't use this transclusion functionality every time that there is a change to the licence, we will need to modify all those pages, which is time consuming and error prone.
Other typical uses for transclusion is for sidebar menus, indices, foooters, page headers and so on, basically any information that you need to include in several documents is better managed in a separated page and transcluded into all the pages where you want that information to be displayed.