If you are in and app and need to save a file into a folder which may be buried deep down in your hierarchy (e.g. Mac HD/Weddings/ 2010/Europe/Spain/The Jonses/Wedding Ceremony/Black and White) it can be a bit of a pain to drill down in the dialogue box, particularly if you have many folder or files in each level and have to scroll down and search each step.
If you have that destination folder open (Black and White in this case) in the Finder in separate window next to your application dialogue box you can just drag the folder from the Finder window into the app dialogue box and it will just jump all the way to that folder without the need to drill down at all! The same works when importing or selecting a file via a dialogue box. I find it saves me so much time and frustration!