(Right now "computers that aren't 100% accurate" are all the rage, even without quantum computing. Though a lot of people are wondering if that's any good, too.)
They're especially good for oracle-type problems, where you can verify an answer much faster than you can find them. NP problems are an especially prominent example of that. If it's wrong, you try again.
In theory it might take a very long time to find the answer. But even if you've only got 25% accuracy, the odds of you being wrong 10 times in a row are only 6%. Being wrong 100 times in a row is a number so small it requires scientific notation (10^-13). It's worth it to be able to solve an otherwise exponential problem.
Quantum computers have error bounds, and you can use that to tune your error rate to being-hit-by-a-cosmic-ray level of acceptability.
It's still far from clear that they can build general-purpose quantum computers big enough to do anything useful. But the built-in error factors are not, in themselves, a bar.
They're especially good for oracle-type problems, where you can verify an answer much faster than you can find them. NP problems are an especially prominent example of that. If it's wrong, you try again.
In theory it might take a very long time to find the answer. But even if you've only got 25% accuracy, the odds of you being wrong 10 times in a row are only 6%. Being wrong 100 times in a row is a number so small it requires scientific notation (10^-13). It's worth it to be able to solve an otherwise exponential problem.
Quantum computers have error bounds, and you can use that to tune your error rate to being-hit-by-a-cosmic-ray level of acceptability.
It's still far from clear that they can build general-purpose quantum computers big enough to do anything useful. But the built-in error factors are not, in themselves, a bar.