I guess those already used to working with any flavor of Unix and X11 have already understood the interest of such a project. In my opinion, anybody who used it for more than a couple of minutes may find it hard to revert to a single desktop. However, some people, most probably Windows users, may not understand what this is.
So, first of all, what is a virtual desktop? Simply said, a "desktop" is what you see when you run windows: the actual windows desktop, with the icons on it; some open windows; some minimized windows. The main goal of the project is thus to allow working with only some of the applications visible at a time. Later, some other windows may be visible. The program thus allows to have a few set of applications/windows, where you can simply choose which group is visible, and switch between one group or the other.
Now, one may ask what is the interest of such a thing ? Well, this is quite simple. If you only have a few windows open at any time, I guess there isn't much interest. However, if you are working with lots of applications, you desktop can easily become overcrowded: you lose time finding your windows, taskbar buttons are too small for you to read the text and find the right window, using drag and drop becomes a nightmare... The solution is Virtual Dimension.
Indeed, using Virtual Dimension, you could have all your chat and irc windows on one virtual desktop; your email client and browser windows on another; and your text editor and debugger on a third one. This way, you only have what you need before you, yet you can easily switch to another desktop when you need to access the other applications.