I would have it coordinate the traffic lights in a city
You don’t need a quantum computer for that.
Quantum computers will be able to efficiently solve a large number of the traditionally NP-complete / intractable problems that standard computers choke on when dealing with a dataset of any worthwhile size.
Downside is that a quantum computer can probably brute force TLS encryption. Upside, however, is that a quantum computer can create encryption schemes that quantum computers will struggle to brute force, so the arms race continues.