Massive decryption powers but also the ability to perform quantum cryptographic tasks which will make it impossible to eavesdrop on encrypted communication without leaving a trace.
There’s a whole lot more that these things can do.
They’ll be able to create materials that can do anything.
More frightening is that they’ll be able to do genetics work on a scale that’s um weird amazing and dangerous. I also think that quantum computers will have the power to questions about quantum space—making warp drive and star travel possible.