Some of your questions are open theoretical questions. There are several ways to answer your question. A general way to think about QM computing is that it harnesses spintronics ie quantum property of spin for computation. So it is a logical next step in the miniaturization of electronics/logic, and computation in general. There are theoretical limits on gate width that are being brushed up against in current fabrication technology, a consequent plateauing of Moores law and spintronics represents the "next frontier".
Spintronics represents a different computing paradigm than binary logic. So it has interesting theoretical properties worthy of exploration even without implementations. However there is a general hope in the field that QM computing is extremely scalable and that once the principles are figured out for a few qubits, the systems could be scaled to many qubits "without too much trouble". Theoretically it is shown to scale in processing complexity in a much different/more dramatic way than classical computing, ie roughly there is $2^x$ processing capability where $x$ is the number of qubits, ie exponential rise in computational capability for linear increase in qubits. This sounds almost like out of science fiction but is an apparently "real/intrinsic" property as far as anyone knows.
A key breakthrough in 1996 is Shor's algorithm, that showed factoring can be solved in "quantum polynmomial time" and it is credited as inciting major interest in quantum computing. Factoring is of course at the heart of modern cryptographic systems in the widely used RSA algorithm.
It is an open theoretical question if quantum computers can solve other major problems in "faster" time. This is known as the BPP=? BQP question.
A controversial QM computer is built by DWave which has been proven to be "useful" in solving some problems, and they have successfully demonstrated a form of quantum scaling on a "somewhat weaker" type of QM system known as adiabatic computing. It is an open question whether it can/ will ever demonstrate unequivocal speed increases, actively under research eg by Google, Nasa, Lockheed etc.
In short quantum computers are not exactly "useful" in the same sense as classical computers, that exact nature of their usefulness is being actively researched, and only limited/ experimental/prototype systems are current in existence. They are conjectured to be "at least as useful" as conventional computation upon their realization, and possibly/hopefully "more useful" in certain not-exactly-foreseeable ways.