But there is still room for doubt about whether one can build them out of actual atoms and make them work in a useful way. Some people are still pessimistic about that, but either way, that debate is not really a scientific one at the moment, because there is no scientific theory about what can and can't be built. Similar questions are raised by the whole range of nanotechnology that has been proposed in principle. So that's where a quantum constructor theory is needed. Because quantum theory is our basic theory of the physical world.
All construction is quantum construction.
Quantum computing is information processing that depends for its action on some inherently quantum property, especially superposition. Other quantum computations, notably quantum cryptography, couldn't be done by classical computers even in theory.