On the practical side:
A lot of work is currently going into getting small scale quantum computers to work, and this involves fundamental understanding and manipulation of small quantum systems, be it ions, photons or whatever else. I can only imagine what other uses we would be able to find for having a better grasp on handling these fundamental systems. A side of effect of the industry of quantum computers is also that a lot of research and development is going into for instance cryogenic cooling. If this means that cooling to low temperatures becomes cheaper or easier this could have a multitude of benefits in other areas. The same holds for any other technology being developed for quantum computers.
On the theoretical side:
There are of course the "quantum-inspired" algorithms that run on classical computers, but would perhaps have been hard to come up with without thinking about these problems trough the lens of a quantum computer (see for instance the work by Ewin Tang on dequantizing certain quantum algorithms). Beyond that I think the concepts of BQP and related complexity classes helps to have a more clear idea in complexity theory of which problems are hard, and exactly how hard they are. And to paraphrase Scott Aaronson, if the future does not contain large scale quantum computers, then this raises the very interesting question of why? What would have stopped us from achieving this? Are there some theoretical boundaries that prevent us from doing so? Because that would certainly be a very interesting topic of research.