Thirty spokes converge upon a single hub;

It is on the hole in the center that the use of the cart hinges.

We make a vessel from a lump of clay;

It is the empty space within the vessel that makes it useful.

We make doors and windows for a room;

But it is these empty spaces that make the room livable.

Thus, while the tangible has advantages, it is the intangible that makes it useful.