This is a 3D design tool for use with programs like Maya, 3D studio, Lightwave, CAD, and Softimage. It is also used with the left (or non-mouse) hand to rotate and maneuver the camera in 3D while the mouse hand is used to select and manipulate polygons etc.
Its basically to cut down the "double duty" of the mouse in these programs, since the mouse manipulates the camera (normally with a mod key like ctrl) and then after positioning it is used to paint or select.
I guess when your job is to produce 3D models and environments quickly in an industry like games or post production, every second counts. But for the average user this thing is not even an option.