This may be slightly extending the topic further than winey intended but I've only recently really started paying attention to the versatility of the paths.
The way I see it is there are 2 paths, the server-side path and the client-side the path.
The client side path is the path that the browser knows, based upon the URL. /images/pic1.gif is an image directly available to the browser.
However the serverside path is one I can use from any serverside script and when you have scripts that include other scripts and you don't know whether the script is ../ or ./ things can get messy.
So to be able to say
include_once("/www/virtual/mysite/htdocs/template.inc");
is invaluable. This obviously won't work from the browser but in php it's great.
An added bonus is that my secure pages are held within the "virtual" folder so I can include content from my insecure site on my secure pages and I don't get a conflict. The html that is passed out is still secure. This means that when I update logic, menus etc I don't have to copy them over to the secure site, I can go straight to testing. (Images and things called by the browser do have to be copied tho').