1 - better blending of edges for cleaner graphics in GUI.
2 - eventually, you may have an RSS reader, or some other information object on the desktop, which you would be able to see through say the corner of a window - allowing you to keep your eyes peeled for information coming into this object, while completing your work in the current application without having to minimize the window to view the information.
3 - you may have a web cam, and if there were a window designed with a transparent section for say an IM, you could easily view the person on the other side of the IM while easily typing at the same time - using maximizing your screen real estate.
4 - you may want to have a music video running while you work, and can see this through the transparency.
5 - you may be testing programs for memory leakage, or CPU usage, and you could set the meter or text based information while running the program at full size in an effort to push the envelope.
6 - you might just like the wallpaper so much, that you want to have that as part of the window title bar.
7 - Why not? 
