This is an experimental demonstration of the technology that makes Neal Stephenson's "Diamond Age" possible. They place an individual carbon atom pair(dimer) on silicon. Silicon has the same crystal structure as diamond, so adding carbon in this manner is similar to building diamond. There is potential for eventually making 3D atomically precise structures of diamond with this tech although that is far off.

This is a very, very basic demonstration, but it's showing for the first time that atoms can be controllably and programmably covalently bonded in something that can probably be extended to 3D. There was a lot of skepticism that this process was possible at all, so an experimental demonstration is important

Inside a vacuum chamber cooled at very low temperatures, they move a scanning tunneling microscopy probe over the surface of a silicon wafer, and with it, in the locations that they choose, they can build some carbon atom structures attached to the silicon surface, atom by atom, in a reproducible way.

This kind of control of atomic structures, where the position of every individual atom can be controlled, is very impressive, but for now this is pure research. Any practical applications are more than a decade in the future.

Techniques derived from this might be used some day either for making some kind of integrated circuits or for making some kind of masks for the production of such circuits, but there is no way to use this directly for the production of some material needed in great quantities, like nanotubes for a space elevator.

For a material needed in great quantities, if you build it atom by atom, there must be billions of such building processes done concurrently, like in a living being, in order to achieve an acceptable growth rate. For now, they use a microscopy probe to attach the atoms one by one and you could not make a sufficient number of these that work concurrently.