While structural tubing (HSS) is for buildings, Mechanical Tubing is for "moving parts" and consumer goods. It is manufactured to much tighter tolerances than standard pipe and is used in everything from exercise equipment and office furniture to telescopic handles and rolls for conveyor belts.
The beauty of mechanical tubing is its versatility. It can be customized by shape, wall thickness, and chemistry to suit a specific manufacturing process whether that involves bending, flaring, or machining. It is the "utility player" of the steel world.