They are usually designed to be used along side a mouse in 3D packages (Soldiworks, Inventor, etc) and allow you to manipulate objects (rotate, translate, zoom) etc.
They aren't designed to be used instead of a mouse.
I have a old serial 4000 FLX, and I haven't used it in a year or two, as most of the time its much faster to just use the mouse (plus they don't support any of the newer versions of cad packages without klugey work arounds).