The TubeTop was designed to make your bike compatible with most mast-style hitch racks to help keep your bike mounted solidly. Oversized diameter tubing and anti-rotation inserts provide greater stability.
It just works. If you have a low top tube, no top tube, a small kid's bike, or a really tight rear suspension, then this thing solves that problem for any kind of suspended trunk or hitch rack. It is invaluable.