Earlier quoted context omitted.
> Why is it useful in a world where no quantum computer physically exists? Turing's and Church's investigations into the various theoretical computing calculi are enormously useful, even though both were done well before the existence of anything we'd recognize as a modern general-purpose computer.
Yeah but you at least had analog forms of things that represented mechanical work through computation (looms, relays, etc...). Nothing exists at all in terms of physical worth in relation to quantum computing and nothing is even close to being _real_ besides a model of a theoretical physical computation that has to be checked by classical means. So I don't really get why there exists a language for it.
In both the classical and the quantum case, building a digital computer (a computer that has the potential to scale to large system sizes, unlike analog computers) is what is difficult.
Babbage and Lovelace imagined a digital computer in 1830s. It took 120 years until there was an actual digital computer in existence. It took the creation of an actual digital computer around 1950 for people to stop saying "digital computers are impossible because of noise". See "Probabilistic Logics and the Synthesis of Reliable Organisms From Unreliable Component" by von Neumann, to see how incredibly surprised theorists were at the time.
Quantum digital computers were conceptualized in the late 1980s, the existence of quantum error correction (the equivalent to von Neumann's paper) was shown in late 1990s. Compared to how long it took for conception-to-realization in the case of classical computers (120 years), the quantum side of things is not that terrible (30 years and counting).