Besides giving a nice preview of articles, it gives immediate access to certain functions (such as edit, move, search, discuss, delete ..). It also has some built-in helpers for fixing links that point to disambiguation pages or redirects (these are Wikipedia-typical problems and not necessarily of interest for other wikis).
You can configure it in various ways; for example, you can define after how many seconds the little “popup” (not really a popup since it does not create an actual browser window) appears. Possible future coolness include support for getting pages from other wikis and cascading popups.
I think similar navigation popup functionality should be added to other wiki engines. That’s why this post is in the “Mindshare” category, a new category for cool implementations that are not widely known. Why not help identify and collect cool [[edit:Wikis|wiki ideas]] in the ID wiki itself?